fix: recursively size variable size structs in your buffer layouts

[steveluscher]

Problem

Imagine you have a data structure like this:

Layout.structure([
  u8('instruction'),
  Layout.structure([
    rustString('variableSizeString'),
  ]);
]);

The present implementation of getAlloc will not dive into that inner structure and size it, so the total size of this entire layout will – incorrectly – be 1.

Summary of Changes

• When getAlloc encounters a structure it recurses into it to obtain its size.

[steveluscher]

This will match both Structure and BitStructure objects, but I did not test it with BitStructure. I presume that BitStructure objects can't be of variable length, so they will match the if (item.span >= 0) check above.

[steveluscher]

I encountered this bug while creating #27627.

[codecov]

Codecov Report

Merging #27624 (1d37805) into master (e779032) will decrease coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #27624     +/-   ##
=========================================
- Coverage    76.9%    76.6%   -0.4%     
=========================================
  Files          48       52      +4     
  Lines        2505     2659    +154     
  Branches      355      366     +11     
=========================================
+ Hits         1927     2037    +110     
- Misses        448      488     +40     
- Partials      130      134      +4