add Range constructors from std::array

add Range constructors from std::array

Summary:
Add explicit constructors to create Range objects referring to the contents of
a std::array.

D4165608 previously added a folly::range() factory function to create Ranges
around const arrays.  However, it did not support non-const arrays.  However,
providing this as a constructor seems better.  This new constructor does not
appear ambiguous with any existing constructors, and a constructor allows for
more natural support of non-const arrays: the caller can explicitly indicate
if they want to construct a Range to const or non-const data in this case.

Reviewed By: yfeldblum

Differential Revision: D4523515

fbshipit-source-id: c1a262f9e8f76907d87d80b03f252576506e70ab