Add missing 'const' on pool_resource& in synchronized_pool_resource constructor.

I'm taking this as editorial because there's no indication that a difference
was intended from unsynchronized_pool_resource or the other
synchronized_pool_resource constructors.

Fixes #22.
Code-font the actual function parameter in "block of size bytes" in [memory.resource.pool.mem].

Fixes #24
Use dynamic_cast<const T*> where the argument is const.

Otherwise the definitions violate the rule against dynamic_cast'ing away const.

Fixes #25