If we ever implement some sort of ABI for dynamic extension loading, we'll need these underlying APIs to support multiple extensions, so we might as well do that first.
These .wrap_<type>() functions aren't supposed to capture resources from the
language instance. It was convenient that wrap_() could be called without fully
spelling the language type, but doing that would introduce lifetime issue in
later patches.
I added type alias L to several places because the language type is usually
called L in generic code.
