Consolidate rt's three definitions of rust_fn

[brson]

The runtime defines rust_fn in three places. This isn't necessary.

[brson]

Also fn_env_pair

[jamorton]

Is this fixed? I see only two definitions, and they don't look like they are for the same thing. And only one definition of fn_env_pair exists these days.

[brson]

There are only two named rust_fn now, but fn_env_pair describes the same structure, so three total. They are all just a function pointer + environment pointer. The struct they all describe is just a pair of void pointers, but they are redefined in different ways based one how they are used in the runtime.

[catamorphism]

This is still an issue: there's only one rust_fn and only one fn_env_pair, but they seem to serve the same purpose.

[luqmana]

This should be fixed in incoming now.