Give C symbols that need to be exported a `_C` suffix so that they can
be linked into a Rust symbol with the correct name (Since we can't
directly export from C/C++ with Rust+Cargo)
See: https://github.com/rust-lang/rfcs/issues/2771
This general Activity glue crate now supersedes the two separate
native-activity and game-activity crates.
The adds the functionality of both crates as modules within this
new crate but provides a single crate + API that downstream can
depend on as an Activity glue layer.
The new crate exposes two (mutually-exclusive) features for
choosing which Activity backend to build:
* "native-activity"
* "game-activity"
This common crate makes it more practical to write code that's
Activity agnostic.
Robert Bragg