Convert statics to constants

Due to [a recent breaking change](https://github.com/rust-lang/rust/commit/90d03d792669fed99b659d1efbe835d4b9b8873c) in the Rust compiler, constant values now have to be labeled with `const` instead of `static`.

This commit causes a few warnings due to the non-caps constants on [line 1034 in lib.rs](https://github.com/Noctune/glfw-rs/blob/a890c8589b97a824f67ac94550ae66bcf3fd6edc/src/lib.rs#L1034), but making them all-caps would be a breaking change.