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.
Previously, libclang was run on each fragment (import "C") separately.
However, in regular Go it's possible for later fragments to refer to
types in earlier fragments so they must have been parsed as one.
This commit changes the behavior to run only one C parser invocation for
each Go file.
