[WIP] Conversion fn prefixes: change to fit Rust's convention

[danielrempel]

Fix #1036

Please review the changes.

I tried to strive to these rules, but I'm not sure about some of the changes I made. Most important to me was to change Surface::as_texture() to Surface::to_texture()

I'm not sure about src/sdl2/joystick.rs-s HatState::as_raw(), because it's owned and I believe it's Copy, too, so according to the table it should be to_raw()?

Prefix	Cost	Ownership
as_	Free	borrowed -> borrowed
to_	Expensive	borrowed -> borrowed
to_	Expensive	borrowed -> owned (non-Copy types)
to_	Expensive	owned -> owned (Copy types)
into_	Variable	owned -> owned (non-Copy types)

[Cobrand]

I agree with everything expect window and surface which IMO shouldn't be changed. The idea in those cases is that they are accessors, and you can do stuff like

canvas.window_mut().set_title("Some title"). Aside from that you can't really "use" SurfaceRef and WindowRef.

This is exactly like io::Error's kind method, and many others taht exist in the std : io::error kind. Notice that the method is called kind() and not as_kind()