Enable find/emplace for key types other than KeyT.

Enable find/emplace for key types other than KeyT.

Summary: Add template parameters to support arbitrary types when looking up a key.

This is useful to avoid the cost of 'materializing' a key which is likely to be present in the array (or, alternatively, switching on a tagged union KeyT).

Use the new capability to greatly simplify the lookup logic in HHVMs static string table.

Reviewed By: nbronson

Differential Revision: D2662451

fb-gh-sync-id: 707fa033f350b80ca8080af17f1a8a74c59f2e88