experiment: add implicit keyword and dedicated grammar

[crusso]

builds on #5517

• add implicit keyword and grammer production (so a minor breaking change)

• expands implicit t to a named type. Since implicit is now a keyword, users can't just use identifier implicit as name for a traditional named type (or anything else for that matter).

• extends pretty printer, parser and various other bits and bobs.

Let's you write and read:

func sort<T>( array : [T], compare : implicit (Nat, Nat) -> Order) {};
: <T>( array : [], compare : implicit (Nat, Nat) -> Order) -> ()

instead of raw syntax and types:

func sort<T>( array : [T], compare : (implicit (Nat, Nat) -> Order)) {};
: <T>( array : [], compare : (implicit : (Nat, Nat) -> Order)) -> ()

(fewer (..) and : )

• merge with master

• fix merge (regen tests)

[ggreif]

I'd suggest

func sort<T>( array : [T], compare : implicit (Nat, Nat) -> Order) {};
: <T>( array : [], with compare : (Nat, Nat) -> Order) -> ()

• spares us from introducing a new keyword
• (to me at least) it reads naturally "with compare : (Nat, Nat) -> Order from callsite lexical context"
• the keyword is right at the beginning
• most programmers will regard implicit as a very exotic word

[crusso]

A regression?

[crusso]

A regression?

[github-actions]

Comparing from 69d8fd4 to 1817b1a:
In terms of gas, no changes are observed in 5 tests.
In terms of size, no changes are observed in 5 tests.