Memoizing in ClassReflector/FunctionReflector/ConstantReflector would help with performance

[ondrejmirtes]

See PoC implementation that uses inheritance for decoration (but it could be done directly in the implementation): https://github.com/phpstan/phpstan-src/blob/master/src/Reflection/BetterReflection/Reflector/MemoizingClassReflector.php

[Ocramius]

I think the entire class/function/constant reflection where the reflectors depend on each other is very messy as it is: this totally requires an architectural change, rather than pushing more responsibility down the reflectors themselves.

[Ocramius]

This is mostly solved by #772.

I suggest that we let userland implement this for now, and later on we can go back to it and add a decorator for Reflector by default later, should it be interesting.