Provide more flexible caching for AAST

Summary:

• Provide buildSelectCache() to improve the performance of multiple selectDescendandsOfType() calls.
• Provide selectTokensOfType() as a performant accessor for some token selection.
• Provide buildTokenCache() to improve the performance of multiple selectTokensOfType() calls.
• Memoize and microoptimize some other hot paths.

Test Plan: XHPASTLinter runs correctly but ~5x faster on the unit test dataset (see next diff).

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1828