Typecast on writing

[aderyabin]

Currently, typecasting works on attribute reading. There are two defects:
– every reading is every typecasting.
– if typecasted class has attributes they can't be changed, because typecasting creates a new instance every time.

I suggest typecasting on writing. It avoids from both defects

[film42]

@aderyabin I think we should actually find a way to move away from creating a new instance of a type caster class because I've found it to add an enormous amount of overhead in object allocation. I started moving over to setting a :typecaster directly, because I found an order of magnitude in speed-up by not creating a new typecaster each time.

EDIT: I should have mentioned, I saw this performance bottleneck while running under a profiler (which is very costly in terms of allocations), but in real world MRI examples, it's about 10% faster (2 second reduction after 10k iterations of a model with 93 attributes).

[aderyabin]

@film42 Could you please share benchmark script?