Swap rules lexer for much faster one.

This swaps github.com/kivikakk/golex for github.com/cznic/golex.

The old lexer would have taken 3.5 years to load a set of 5000 test rules
(quadratic time complexity for input length), whereas this one takes only 32ms.
Furthermore, since the new lexer is embedded differently, this gets rid of the
global parser variables and makes the rule loader fully reentrant without a
lock.
Merge pull request #327 from prometheus/optimize/faster-lexer

Swap rules lexer for much faster one.