Remove moduleContextImpl struct

The moduleContextImpl structure contains duplicate information on the
contexts. BaseModuleContext, ModuleContext and DepsContext can already
surface which Module is being processed via Module(). Because most of
the Rust-specific methods on the *Context structures simply forward to
the Module methods, expose a RustModule() method.

Test: cd external/rust/crates; mma
Change-Id: Ifee90825d54081fc5e9a8df0b7c4580412e9158c