Improvement - get_key_pressed being mutable reference

[PerfectlyFineCode]

Hi!

I stumbled upon this interesting detail that get_key_pressed is mutable, which means it breaks some behaviors.
get_gamepad_button_pressed isn't mutable, so why is get_key_pressed?

I made an extended function which calls exactly the same thing and it doesn't need to be mutable at all.
Code of said issue:
https://github.com/deltaphc/raylib-rs/blob/master/raylib/src/core/input.rs#L36
where it is a mutable reference to self, as no mutability is required. This can be removed.