Alias all functions and symbols to a single namespace

[adetaylor]

Right now, to use autocxx-generated bindings from Rust, you need to refer to:

• ffi::cxxbridge::W for struct W
• ffi::cxxbridge::X for struct X
• ffi::Y for constant Y
• ffi::defs::Z for preprocessor symbol Z

(I think).

This all needs tidying up; at least the first two should be aliased so you can refer to them using just ffi::W; ffi::Z.

It might be that ffi also needs to be made changeable in case there are multiple include_cpp macros in a file.

[adetaylor]

C++ namespace support is gradually landing, but currently consists purely of understanding the C++ side of things without attempting to replicate the same namespaces on the Rust side.

Partly I've not done that because we need to finish dtolnay/cxx#353, and partly because we need to finish this item first as well.