py/emitglue: Introduce mp_proto_fun_t as a more general mp_raw_code_t.

Allows bytecode itself to be used instead of an mp_raw_code_t in the simple
and common cases of a bytecode function without any children.

This can be used to further reduce frozen code size, and has the potential
to optimise other areas like importing.

Signed-off-by: Damien George <damien@micropython.org>

docs/develop/compiler.rst

@@ -157,7 +157,7 @@ The most relevant method you should know about is this:
        mp_compile_to_raw_code(parse_tree, source_file, is_repl, &cm);
 
        // Create and return a function object that executes the outer module.
-       return mp_make_function_from_raw_code(cm.rc, cm.context, NULL);
+       return mp_make_function_from_proto_fun(cm.rc, cm.context, NULL);
    }
 
 The compiler compiles the code in four passes: scope, stack size, code size and emit.