Summary:
F14 maps and sets were missing the operator= overload that takes
an initializer list, resulting in map = {} having reset behavior rather
than clear behavior.  Without the overload an empty map is constructed and
then moved onto the lhs, freeing all memory.  With the overload we call
clear(), which retains the existing allocation when it is not too large.
This diff also cleans up the default constructor definitions, adding
noexcept for F14FastMap and F14FastSet and making the others more concise.

Reviewed By: shixiao

Differential Revision: D9661526

fbshipit-source-id: 101d66623bc4d50e7935fa9a73405a825648e6a5