Manually inline unserializeKey

Summary: This just bounces execution from type-variant to variable-unserializer
and back for no really good reason.  It's only used in two places, so just do
it inline.

Reviewed By: @jdelong, @alexmalyshev

Differential Revision: D1747600

Signature: t1:1747600:1418869968:22e2abe342d2cd4730fb58a953096b25144da6ff