Performance tweaks & hex_active_theme API change

I did some profiling and there were parts that were calling functions very often that weren't needed. The hex theming context was removed and rolled into hex_active_theme where it was originally.

hex_active_theme was called every time the cache API was called. It was moved to a global 'theme_target'. hex_alter and hex_cache now uses the fast static caching pattern used by core.

hex_cache now returns an object straight from cache_get.