Reduced need of unsafe in FFI

[jorgecarleitao]

This PR simplifies the API of FFI:

• allocating the FFI structs on the heap is no longer required
• exporting is now safe since we only need the struct to be on the stack
• use Box on Bytes, further simplifying this core struct.
• use write instead of write_unaligned
• cleaup the integration test code

Backward incompatible changes

We no longer require a pointer to export - just export the struct to the stack and Box it if the other end of the interface requires a pointer. If the other end is the one allocating an empty struct, then write the exported struct to the pointer (which is unsafe as the receiving end must produce a valid pointer ^^).

[codecov]

Codecov Report

Merging #1100 (69a9f9b) into main (88f05bb) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1100      +/-   ##
==========================================
+ Coverage   81.24%   81.25%   +0.01%     
==========================================
  Files         366      366              
  Lines       35336    35337       +1     
==========================================
+ Hits        28707    28713       +6     
+ Misses       6629     6624       -5     

Impacted Files	Coverage Δ
src/buffer/bytes.rs	98.11% <100.00%> (ø)
src/ffi/array.rs	89.67% <100.00%> (+0.47%)	⬆️
src/ffi/mod.rs	100.00% <100.00%> (ø)
src/ffi/stream.rs	67.80% <100.00%> (ø)
src/io/ipc/read/reader.rs	95.41% <0.00%> (-0.77%)	⬇️
src/array/utf8/mod.rs	82.74% <0.00%> (+0.95%)	⬆️
src/chunk.rs	90.47% <0.00%> (+7.14%)	⬆️

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