Add repr(C) attribute to PurpleDebugLevel

This silences the ffi-unsafe enum lint, and is probably a good idea (tm)
besides.

Signed-off-by: Anton Lofgren <anton.lofgren@gmail.com>
Add purple_debug ffi

So, this is unsafe, and I'm not going to expose it through the "safe"
API, since there's currently no way in Rust to create variadic
functions.

I'm also merging the debug tests into a single big one, since otherwise
they're prone to breaking each other.

Signed-off-by: Anton Lofgren <anton.lofgren@gmail.com>