Refactor build script (and feature flags)

[nbigaouette]

• Since bindgen generated files are platform dependants, they are moved to their own directory onnxruntime-sys/src/generated/<os>/<arch>/bindings.rs.
• Feature flags reworked:
  • disable-sys-build-script: Disable build script, used only for docs.rs
  • generate-bindings: Generate the bindings using bindgen. This removes the need for its compilation (since bindings are committed).
• To generate the bindings (f.e. on version update or different platform): cargo build --package onnxruntime-sys --features generate-bindings

[codecov-io]

Codecov Report

Merging #31 into master will decrease coverage by 6.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
- Coverage   19.18%   13.01%   -6.17%     
==========================================
  Files          17       18       +1     
  Lines         610      899     +289     
==========================================
  Hits          117      117              
- Misses        493      782     +289     

Impacted Files	Coverage Δ
...runtime-sys/src/generated/macos/x86_64/bindings.rs	0.00% <ø> (ø)

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