If bindgen finds an inlined function and the
`--generate-extern-functions` options is enabled, then:

- It will generate two new source and header files with external
  functions that wrap the inlined functions.
- Rerun `Bindings::generate` using the new header file to include these
  wrappers in the generated bindings.

The following additional options were added:
- `--extern-function-suffix=<suffix>`: Adds <suffix> to the name of each
  external wrapper function (`__extern` is used by default).
- `--extern-functions-file-name=<name>`: Uses <name> as the file name
  for the header and source files (`extern` is used by default).
- `--extern-function-directory=<dir>`: Creates the source and header
  files inside <dir> (`/tmp/bindgen` is used by default).

The C code serialization is experimental and only supports a very
limited set of C functions.

Fixes rust-lang#1090.

test(static inlined): pretty print the diff

Issue: rust-langGH-1090

test(static inlined): refactor paths once again

- Because directory structure is confusing to the author of this commit

rust-langGH-1090

test(static inlined): refactor test files

- Expected files should be under tests/expectations/generated
- Remove extern_stub.h because we can reuse the header from generate-extern-functions.h

test(static inlined): diff test; generated files

refactor(static inlined): integration test

Issue: rust-langGH-1090

rust-langGH-1090: Integration tests

integration test: document the GH issue

integration test (macro): same order in impl as the trait for consistency

integration test: move each setup into its own function, just to save face

inline static func integration test

resolve accidental conflict