Add StringKeyed(Unordered){Set,Map} to folly
Summary: We're using StringKeyed* from common/datastruct to avoid unnecessary string creation whenever we're looking up string keys. C++14 does offer a solution, see e.g. http://stackoverflow.com/questions/
10536788/avoiding-key-construction-for-stdmapfind. That is not supported by current compilers.
Test Plan: unittests
Reviewed By: pavlo@fb.com
Subscribers: trunkagent, net-systems@, folly-diffs@, yfeldblum
FB internal diff:
D1825700
Signature: t1:
1825700:
1423086724:
530550c3c80e33c80900f31c0ade05c66b22cbe8