This patch replaces single-char L type aliases with P, and renames the L aliases
where that makes sense. Many of the P aliases will be removed later by
introducing generic wrap<T>() trait.
I'm trying to refactor property wrapping functions, and noticed that it's odd
that .wrap_<property>() does boxing internally whereas .wrap_template() doesn't.
Also, it sometimes makes sense to turn property into trait object earlier. For
example, we can deduplicate L::wrap_boolean() in build_binary_operation().
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.
