Feature / BC break: Add support for getMultiple(), setMultiple(), deleteMultiple(), clear() and has() supporting multiple cache items (inspired by PSR-16). (#32 by @krlv and #37 by @clue)

Documentation for TTL precision with millisecond accuracy or below and use high-resolution timer for cache TTL on PHP 7.3+. (#35 and #38 by @clue)

Improve API documentation and allow legacy HHVM to fail in Travis CI config. (#34 and #36 by @clue)

Prefix all global functions calls with \ to skip the look up and resolve process and go straight to the global function. (#31 by @WyriHaximus)