py/compile: Add option to allow compiling top-level await.

Enabled by MICROPY_COMPILE_ALLOW_TOP_LEVEL_AWAIT.  When enabled, this means
that scope such as module-level functions and REPL statements can yield.
The outer C code must then handle this yielded generator.

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

py/compile.h

@@ -30,6 +30,11 @@
 #include "py/parse.h"
 #include "py/emitglue.h"
 
+#if MICROPY_COMP_ALLOW_TOP_LEVEL_AWAIT
+// set to `true` to allow top-level await expressions
+extern bool mp_compile_allow_top_level_await;
+#endif
+
 // the compiler will raise an exception if an error occurred
 // the compiler will clear the parse tree before it returns
 // mp_globals_get() will be used for the context