replace nex with ad hoc lexer

Nex generates tokens by spawning an scanning thread which uses two channels and a whole host of anonymous functions to implement as set of states for a rather complicated finite state machine.
Token scanning is currently the most expensive part of parsing, is GC intensive, and has been known in the past to leak goroutines.

implement a much simpler ad hoc lexer which identifies the next token at each call and is heap neutral.