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)
Merge branch 'develop' of /Users/neko/github/CodeIgniter/user_guide_src_3/ into develop_japanese