Adding a custom lexer for asm.js parsing.
It takes advantage of a number of asm.js properties to simply things:
* Assumes 'use asm' is the only string.
* Does not handle unicode for now (tools don't emit it).
* Combines global + local string table with lexer.

R=marja@chromium.org,vogelheim@chromium.org,kschimpf@chromium.org

BUG=v8:4203
BUG=v8:6090

Review-Url: https://codereview.chromium.org/2751693002
Cr-Commit-Position: refs/heads/master@{#43874}