Consistently use unsigned/size_t in big-int [blocks: #2310]

[tautschnig]

No description provided.

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: a323dd2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/77183230
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

[kroening]

The index shifts are certainly scary.

Equally scary is the idea that most of these 'unsigned' should likely be size_t, even though that means that we worry about an integer that needs more than 2^32 bits to represent.

[allredj]

Could we have unit tests for these, to make sure that we're not breaking any corner case?

[tautschnig]

I have added a commit to move to size_t and have also enabled a unit test for BigInt.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e5501eb).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/91047441

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: d63c049).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/91344288

[NlightNFotis]

Hi @tautschnig, is this something you plan on merging soon?

If you'd like it merged but don't have the time to work on this, is there something I can do to get this in a better shape to merge?

[TGWDB]

Closing due to age (no further comment on PR content), please reopen with rebase on develop if you intent to continue this work.

[tautschnig]

@kroening This has been rebased and I hope to have addressed all past concerns.

[codecov]

Codecov Report

Attention: Patch coverage is 92.91339% with 9 lines in your changes missing coverage. Please review.

Project coverage is 79.59%. Comparing base (66004dc) to head (c7a1826).

Files with missing lines	Patch %	Lines
src/big-int/bigint.cc	92.62%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2452      +/-   ##
===========================================
+ Coverage    78.71%   79.59%   +0.88%     
===========================================
  Files         1732     1732              
  Lines       199536   197321    -2215     
  Branches     18281    18214      -67     
===========================================
+ Hits        157057   157059       +2     
+ Misses       42479    40262    -2217     

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