derive(IntoBytes): support repr(C) structs with explicit trailing slices

[jswrenn]

For example:

#[repr(C)]
struct Example {
    leading: u8,
    trailing: [Trailing]
}

Makes progress towards #1112

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.50746% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.91%. Comparing base (3d35f06) to head (33c3fd7).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/layout.rs	97.46%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2679      +/-   ##
==========================================
+ Coverage   88.75%   88.91%   +0.16%     
==========================================
  Files          20       20              
  Lines        5343     5422      +79     
==========================================
+ Hits         4742     4821      +79     
  Misses        601      601              

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[joshlf]

Wooo!!!

Can you add UI tests that succeed but wouldn't have succeeded before this change? We may also want some new failure ones to make sure we're properly exercising the edge cases of our logic (namely, what if you have a non-syntactically-DST-but-actually-DST repr(C) type? What about a syntactically-DST non-repr(C) type? etc).

[jswrenn]

Can you add UI tests that succeed but wouldn't have succeeded before this change? We may also want some new failure ones to make sure we're properly exercising the edge cases of our logic (namely, what if you have a non-syntactically-DST-but-actually-DST repr(C) type? What about a syntactically-DST non-repr(C) type? etc).

Done