Deprecate CBOR_CUSTOM_ALLOC

[PJK]

Description

CBOR_CUSTOM_ALLOC was a mistake, the defaults with CBOR_CUSTOM_ALLOC=ON are compatible with CBOR_CUSTOM_ALLOC=OFF.

I would like to remove it going forward. Will keep this PR open for a few weeks to give people a chance to object.

Pros of having the flag:

• Clients with simple use cases can (maybe, in practice likely optimized away) save one function resolution indirection

Cons:

• Complexity everywhere (code, build config, tests, distribution)
• Possibility of incompatible (esp. shared) libraries

Also reworked the bazel build example since it was miswired and broke due to the flag change

Checklist

• I have read followed CONTRIBUTING.md
  • I have added tests
  • I have updated the documentation
  • I have updated the CHANGELOG
• Are there any breaking changes?
  • If yes: I have marked them in the CHANGELOG (example)
• Does this PR introduce any platform specific code?
• Security: Does this PR potentially affect security?
• Performance: Does this PR potentially affect performance?

[codecov-commenter]

Codecov Report

Merging #237 (c1032ce) into master (3dfc2d5) will not change coverage.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files          20       20           
  Lines        1543     1543           
=======================================
  Hits         1478     1478           
  Misses         65       65           

Impacted Files	Coverage Δ
src/cbor/internal/builder_callbacks.c	89.28% <50.00%> (ø)
src/cbor/serialization.c	85.16% <50.00%> (ø)
src/cbor/bytestrings.c	94.64% <75.00%> (ø)
src/cbor/arrays.c	98.24% <100.00%> (ø)
src/cbor/common.c	100.00% <100.00%> (ø)
src/cbor/floats_ctrls.c	96.66% <100.00%> (ø)
src/cbor/internal/memory_utils.c	93.75% <100.00%> (ø)
src/cbor/internal/stack.c	100.00% <100.00%> (ø)
src/cbor/ints.c	99.00% <100.00%> (ø)
src/cbor/maps.c	96.36% <100.00%> (ø)
... and 2 more

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