Use NonNull in Allocator

[stepancheg]

This matches API of std allocator https://doc.rust-lang.org/std/alloc/trait.Allocator.html

But more broadly speaking, in capnp-rust codebase, some pointers are used with zero is information, and in other places pointer being zero is not valid. might be better to convert all the pointers to NonNull, and use Option where null value is expected.

All pointers start with allocator, so this PR.

[codecov]

Codecov Report

❌ Patch coverage is 82.92683% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.00%. Comparing base (ab342b3) to head (d9229c0).
⚠️ Report is 280 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/message.rs	80.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #638      +/-   ##
==========================================
- Coverage   51.64%   51.00%   -0.65%     
==========================================
  Files          69       71       +2     
  Lines       33735    32515    -1220     
==========================================
- Hits        17422    16583     -839     
+ Misses      16313    15932     -381     

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