Align PHPAST lexer to PHP 5.5.8

Summary:
Ref T4334. This aligns the lexer (but not the parser) with PHP 5.5.8. It also applies the patch on T3512. This has these positive effects:

• Fixes binary literals.
• Fixes empty heredoc.
• Corrects T_BINARY_CAST into T_STRING_CAST (consistent with 5.5.8).
• Removes T_UNICODE_CAST (not in 5.5.8).
• Recognizes keywords trait, __trait__, yield, insteadof, callable.

...these sideways effects:

• Slightly improves terminal heredocs (previously: parse error; now: bad parse).

...and these negative effects:

• Breaks the callable test, since callable is now recognized as a keyword but the parser doesn't yet support it.

Test Plan: Rebuilt xhpast, ran unit tests.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D8032