src: simplify and fix FFI ArrayBuffer accesses

[addaleax]

ArrayBufferViewContents supports ArrayBuffers, so we do not need to provide special treatment for those.

Also, small typed arrays may not have backing stores until their corresponding ArrayBuffers are accessed, so do that in order to avoid accidentally accessing arbitrary stack memory.

[codecov]

Codecov Report

❌ Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.62%. Comparing base (e02087c) to head (b6651d8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ffi/data.cc	65.00%	2 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62857      +/-   ##
==========================================
- Coverage   89.64%   89.62%   -0.03%     
==========================================
  Files         706      706              
  Lines      219137   219124      -13     
  Branches    41999    41981      -18     
==========================================
- Hits       196453   196386      -67     
- Misses      14574    14619      +45     
- Partials     8110     8119       +9     

Files with missing lines	Coverage Δ
src/ffi/data.cc	73.63% <65.00%> (+0.34%)	⬆️

... and 30 files with indirect coverage changes

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