Convert static to const in ffi

Fallout of RFC 69:

https://github.com/rust-lang/rfcs/blob/master/active/0069-const-vs-static.md

The constant values previously used with `static` should now be `const`. Static
is for global variables.

Closes #17.
Merge pull request #18 from tomassedovic/static-to-const

Convert static to const in ffi