cgo: refactor

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.

cgo/testdata/errors.go

@@ -27,7 +27,7 @@ import "C"
 //line errors.go:100
 var (
 	// constant too large
-	_ C.uint8_t = 2 << 10
+	_ C.char = 2 << 10
 
 	// z member does not exist
 	_ C.point_t = C.point_t{z: 3}