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