dynamic constructor from anything String-like
dynamic constructor from anything String-like

Summary:
With the exception of std::string, nothing else string-like can be
optimized further than a copy. So, create a generic Stringish constructor
that copies .size() elements from .data(), where .data() has a value type
of char.

Reviewed By: yfeldblum

Differential Revision: D5407445

fbshipit-source-id: acf709f07e73bb5bef3f124169c81d76edefa52c

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter