Replaced static declarations with const declarations

Version 0.12 of Rust has changed the restrictions on using static variables. I think this is related to [RFC_69](
https://github.com/rust-lang/rfcs/blob/master/active/0069-const-vs-static.md). I've replaced all the static declarations in modules that wouldn't compile to now be constant declarations, and the library now seems to work fine with the current master branch of rust.