Rename yySymType to lexer; should eventually capture all lexer state.
Embed lexer in parser and access lexer token data directly.

Change-Id: I246194705d594f80426f3ba77d8580af9185daf7
Reviewed-on: https://go-review.googlesource.com/19759Reviewed-by: Matthew Dempsky <mdempsky@google.com>