Expose library component

It'd be super nice if Alex exposed some of the types it uses so that any functions working with them don't have to be included directly into the lexers.

For example, in Yi wi have to `#include` https://github.com/yi-editor/yi/blob/master/yi/src/library/Yi/Lexer/common.hsinc in every lexer we want to use the functions in: if we try to move them to a regular Haskell module, we don't have the types in scope when necessary.