feat: Mermaid renderer for hugrs

[aborgna-q]

Adds a HugrView::mermaid_string which produces things like

graph LR
    subgraph 0 ["(0) DFG"]
        direction LR
        1["(1) Input"]
        1--0:0-->3
        1--1:1-->3
        2["(2) Output"]
        3["(3) test.quantum.CX"]
        3--0:1-->4
        3--1:0-->4
        3-.2:2.-4
        4["(4) test.quantum.CX"]
        4--0:0-->2
        4--1:1-->2
    end

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Note that edges in mermaid are unordered, so I had to add the port indices explicitly.

The new code in src/hugr/views/render.rs is just moved from src/hugr/views.rs.

Closes #696

Requires CQCL/portgraph#125

[aborgna-q]

Here's a render for a CFG:

graph LR
    subgraph 0 ["(0) CFG"]
        direction LR
        subgraph 2 ["(2) DataflowBlock"]
            direction LR
            3["(3) Input"]
            3--0:0-->5
            4["(4) Output"]
            5["(5) MakeTuple"]
            5--0:0-->6
            6["(6) Tag"]
            6--0:0-->4
        end
        2--0:0-->7
        2--1:0-->1
        1["(1) ExitBlock"]
        subgraph 7 ["(7) DataflowBlock"]
            direction LR
            8["(8) Input"]
            8--0:1-->9
            9["(9) Output"]
            10["(10) const:sum:{tag:0, val:const:seq:{}}"]
            10--0:0-->11
            11["(11) LoadConstant"]
            11--0:0-->9
        end
        7--0:0-->1
    end

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I also tried adding the type to the edge labels. It's a bit more noisy, but probably more useful too.

graph LR
    subgraph 0 ["(0) CFG"]
        direction LR
        subgraph 2 ["(2) DataflowBlock"]
            direction LR
            3["(3) Input"]
            3--"0:0<br>usize"-->5
            4["(4) Output"]
            5["(5) MakeTuple"]
            5--"0:0<br>Tuple([usize])"-->6
            6["(6) Tag"]
            6--"0:0<br>Sum([Tuple([usize]), Tuple([usize])])"-->4
        end
        2--"0:0"-->7
        2--"1:0"-->1
        1["(1) ExitBlock"]
        subgraph 7 ["(7) DataflowBlock"]
            direction LR
            8["(8) Input"]
            8--"0:1<br>usize"-->9
            9["(9) Output"]
            10["(10) const:sum:{tag:0, val:const:seq:{}}"]
            10--"0:0<br>Sum(UnitSum(1))"-->11
            11["(11) LoadConstant"]
            11--"0:0<br>Sum(UnitSum(1))"-->9
        end
        7--"0:0"-->1
    end

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And this is now the first example, with type info:

graph LR
    subgraph 0 ["(0) DFG"]
        direction LR
        1["(1) Input"]
        1--"0:0<br>qubit([])"-->3
        1--"1:1<br>qubit([])"-->3
        2["(2) Output"]
        3["(3) test.quantum.CX"]
        3--"0:1<br>qubit([])"-->4
        3--"1:0<br>qubit([])"-->4
        3-."2:2".->4
        4["(4) test.quantum.CX"]
        4--"0:0<br>qubit([])"-->2
        4--"1:1<br>qubit([])"-->2
    end

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[codecov]

Codecov Report

Attention: Patch coverage is 98.90110% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.13%. Comparing base (697e7d7) to head (deabfd7).
Report is 2 commits behind head on main.

❗ Current head deabfd7 differs from pull request most recent head f39a14e. Consider uploading reports for the commit f39a14e to get more accurate results

Files	Patch %	Lines
src/hugr/views/render.rs	98.27%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
+ Coverage   84.04%   84.13%   +0.09%     
==========================================
  Files          77       78       +1     
  Lines       14234    14261      +27     
  Branches    14234    14261      +27     
==========================================
+ Hits        11963    11999      +36     
+ Misses       1435     1427       -8     
+ Partials      836      835       -1     

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[aborgna-q]

Here's the CFG rendered in the new test:

graph LR
    subgraph 0 ["(0) CFG"]
        direction LR
        subgraph 2 ["(2) DataflowBlock"]
            direction LR
            3["(3) Input"]
            3--"0:0<br>usize"-->5
            4["(4) Output"]
            5["(5) MakeTuple"]
            5--"0:0<br>Tuple([usize])"-->6
            6["(6) Tag"]
            6--"0:0<br>Sum([Tuple([usize]), Tuple([usize])])"-->4
        end
        2-."0:0".->7
        2-."1:0".->1
        1["(1) ExitBlock"]
        subgraph 7 ["(7) DataflowBlock"]
            direction LR
            8["(8) Input"]
            8--"0:1<br>usize"-->9
            9["(9) Output"]
            10["(10) const:sum:{tag:0, val:const:seq:{}}"]
            10--"0:0<br>Sum(UnitSum(1))"-->11
            11["(11) LoadConstant"]
            11--"0:0<br>Sum(UnitSum(1))"-->9
        end
        7-."0:0".->1
    end

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you can optionally hide the node indices, types, and ports now;

graph LR
    subgraph 0 ["CFG"]
        direction LR
        subgraph 2 ["DataflowBlock"]
            direction LR
            3["Input"]
            3-->5
            4["Output"]
            5["MakeTuple"]
            5-->6
            6["Tag"]
            6-->4
        end
        2-.->7
        2-.->1
        1["ExitBlock"]
        subgraph 7 ["DataflowBlock"]
            direction LR
            8["Input"]
            8-->9
            9["Output"]
            10["const:sum:{tag:0, val:const:seq:{}}"]
            10-->11
            11["LoadConstant"]
            11-->9
        end
        7-.->1
    end

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