This unifies both implementations of the toplevel under a common interface.

Similar Makefile hacks to those in place for dynlink are used: byte
and native implementations are in separate directories, while the
shared interface are in the parent and copied as needed by the build
rules.

This is a preliminary step to remove duplication as much as possible
in the two implementations, but should already be pretty useful for
libraries that use the `Toploop` interface.

the source file location path changed...

Copying the file separately causes issues (there would be two paths to build
`toplevel/byte/toploop.cmi`); this feels a bit cleaner, but the rules are not
guarded anymore; it is ok because of the dependency rules ensuing from our
`beforedepend` rule.