cmd/compile: move lexer into separate file (cleanup)

This is really moving all the non-lexer pieces out of lex.go
into main.go. It's always been confusing that the top-most
compiler entry point (Main) is in the same file with the
lexer. Both files remain of substantial size (> 1000 lines),
which justifies this even more.

No other changes.

Change-Id: I03895589d5e3cc2340580350bbc1420539893dfc
Reviewed-on: https://go-review.googlesource.com/20601
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>