Expose retro_set_environment in RetroCore?

[InquisitiveCoder]

Certain uses of the environment callback can only be done during retro_set_environment; however, users of the crate lack the ability to execute code during that callback. Out of several such options, the libretro_core! macro only handles SET_SUPPORT_NO_GAME. However, this approach requires updating the macro and trait every time a new option is added to the environment callback that can only be handled from retro_set_environment. A more flexible approach that'd support all current and future options would be to add a set_environment() function to RetroCore which the macro then calls.