Add hash_equals() to ext/hash compat layer

Introduced in PHP 5.6 Beta 1 (unfortunately, still undocumented).
RFC: https://wiki.php.net/rfc/timing_attack

(Yes, I am aware that the RFC talks about hash_compare(), the function was later renamed in the implementation.)
[ci skip] Add a note about QB escaping field names
Typo boolen => boolean
[ci skip] Manually apply an improved version of PR #3034 (doc fixes)
Update profiling.rst

There is some confusion about how to enable profiling for queries. This will save some time for devs who want to debug their generated SQL.
Update profiling.rst

correcting rst syntax
[ci skip] Update changelog