Recycle heap on assignment

Recycle heap on assignment

Summary: For standard containers, assignment tries to reuse heap space.
Dynamic assignment does not - it calls destroy(), and then reconstructs
a new dynamic. In the case that the old and new types are the same (eg
assigning a dynamic-vector to a dynamic-vector) we can call through to
the underlying type's assignment operator.

Reviewed By: @jdelong

Differential Revision: D2148093