[MRG] Update and fix cbindgen config and CI

[luizirber]

This PR add some config options for cbindgen to account for some changes to the default C header format it generates.

Also updates cbindgen from 0.14 to 0.19 in CI.

[luizirber]

Ready for merge @ctb @erikyoung85

[codecov]

Codecov Report

Merging #1473 (7fccbf7) into latest (d45baba) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           latest    #1473   +/-   ##
=======================================
  Coverage   89.58%   89.58%           
=======================================
  Files         122      122           
  Lines       18989    18989           
  Branches     1455     1455           
=======================================
  Hits        17011    17011           
  Misses       1750     1750           
  Partials      228      228           

Flag	Coverage Δ
python	94.80% <ø> (+5.21%)	⬆️
rust	67.37% <ø> (-27.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/core/src/index/mod.rs	61.53% <0.00%> (ø)
src/core/src/index/linear.rs	17.30% <0.00%> (ø)
src/core/src/ffi/hyperloglog.rs	0.00% <0.00%> (ø)
src/core/tests/test.rs	100.00% <0.00%> (ø)
src/core/src/ffi/utils.rs	0.00% <0.00%> (ø)
src/core/src/ffi/nodegraph.rs	0.00% <0.00%> (ø)
src/core/src/encodings.rs	77.77% <0.00%> (ø)
src/core/src/index/bigsi.rs	89.47% <0.00%> (ø)
src/core/src/signature.rs	59.39% <0.00%> (ø)
src/core/src/index/search.rs	100.00% <0.00%> (ø)
... and 17 more

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[ctb]

Let's wait 'til tests pass before merging.