Add a way to optimize operations that require Context and Isoalte been entered

[pinepain]

Entering already entered Isolate and/or Context is much cheaper, (200% cheaper). Before that we used to wrap a code that did intensive entering and leaving into FunctionObject callback, which is also overhead and dirty hack. With something like Isolate::withing(callable $callback) and Context::withing(callable $callback) it should be more straightforward and much cleaner to let users to do potential optimizations.