v4.93.0: support HHVM 4.93, new functions, deprecations

• New functions:
  • Math\almost_equals
  • Str\replace_every_nonrecursive
  • Str\replace_every_nonrecursive_ci
• Breaking changes:
  • C\first_async was deprecated. Instead of await C\first_async(some_async_function()), use C\first(await some_async_function()).
  • C\firstx_async was deprecated (as above).
  • PseudoRandom\float and SecureRandom\float now return values from the half-open interval [0.0, 1.0) (previously [0.0, 1.0], i.e. the value 1.0 could have been returned but no longer can)
• HHVM compatibility fixes:
  • Uses the new function reference syntax instead of fun(...).
  • Regex functions are now pure (they no longer have the side-effect of setting the global error code).
  • Lists of contexts/capabilities were added to most function headers, so they can be called from other code that uses contexts/capabilities.