CLDR plural parser in PHP

CLDR plural parser in PHP

Wrote a CLDR plural rule parser to replace the eval()-based one from
I58a9cdfe. It converts the infix notation of the XML files to a
sanitized RPN notation, referred to in external interfaces as the
"compiled" form. The RPN notation is cached and then executed by a
fast non-validating evaluator.

Timings for the largest rule in the XML file are ~1.2ms for
compilation and ~200us for execution.

Also:
* Lazy-load the plural rules when recache() requests them, instead of
  loading them for every request.
* Language::convertPlural() needs integer keys, and CLDR only gives
  string keys. The previous code was not mapping them so it didn't work
  at all. I just mapped them in the order they appear in the XML file,
  i.e. the first rule becomes MediaWiki's $pluralForm=0, the second
  becomes $pluralForm=1, etc. Not sure if there is a more rigorous way
  to do it.

Change-Id: I65ee788c1a8e5ee2ede2091990d86eb722749dd3