This is a large refactor of the cgo package. It should fix a number of
smaller problems and be a bit more strict (like upstream CGo): it for
example requires every Go file in a package to include the header files
it needs instead of piggybacking on imports in earlier files.
The main benefit is that it should be a bit more maintainable and easier
to add new features in the future (like static functions).
This breaks the tinygo.org/x/bluetooth package, which should be updated
before this change lands.
This implementation is still very limited but provides a base to build
upon. Limitations:
* CGO_CFLAGS etc is not taken into account.
* These CFLAGS are not used in C files compiled with the package.
* Other flags (CPPFLAGS, LDFAGS, ...) are not yet implemented.
