Require Send+Sync bounds for Allocation trait

[jhorstmann]

Which issue does this PR close?

Closes #1944.

Rationale for this change

Send + Sync were previously implemented for Buffer which probably was not sound in case the underlying Allocation was not also Send + Sync.

What changes are included in this PR?

• Allocation trait requires Send + Sync
• Bytes implements Send + Sync which should be fine since it only contains a NonNull pointer in addition to the allocation
• Buffer is then automatically Send + Sync
• FFI_ArrowArray now has to implement Send + Sync. This requirement was previously hidden by the unsafe impl on Buffer, even before the introduction of the Allocation abstraction in Decouple buffer deallocation from ffi and allow creating buffers from rust vec #1494

Are there any user-facing changes?

This could be a breaking change for users of the Allocation trait, but those usages would have been unsound before.

[codecov-commenter]

Codecov Report

Merging #1945 (22b6017) into master (9f7b600) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1945      +/-   ##
==========================================
- Coverage   83.48%   83.47%   -0.01%     
==========================================
  Files         221      221              
  Lines       57054    57054              
==========================================
- Hits        47629    47627       -2     
- Misses       9425     9427       +2     

Impacted Files	Coverage Δ
arrow/src/alloc/mod.rs	78.04% <ø> (ø)
arrow/src/buffer/immutable.rs	98.48% <ø> (ø)
arrow/src/bytes.rs	73.07% <ø> (ø)
arrow/src/ffi.rs	86.97% <ø> (ø)
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.46%)	⬇️

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[alamb]

I re-read https://doc.rust-lang.org/nomicon/send-and-sync.html and this change makes sense to me (push the Send / Sync validation to the Allocation

cc @tustvold and @viirya

+1 for cleaning up clippy lint