Faster accessors for dynamic values

Faster accessors for dynamic values

Summary:
Sometimes it is useful to have a fast accessor to a value stored in dynamic, without any type conversion. These still do the type check, but compiler has more room for optimization.

Test Plan: build folly, will add unit tests/benchmarks if this makes sense at all.

Reviewed By: njormrod@fb.com

Subscribers: jdelong, njormrod

FB internal diff: D1564867