add Cursor::peekBytes()

add Cursor::peekBytes()

Summary:
The existing Cursor::peek() method pre-dates the existence of folly::ByteRange,
and so it returns a std::pair containing a pointer and length.  This is usually
more awkward to use than ByteRange.

This adds a peekBytes() method that returns a ByteRange object, and updates all
users of peek() in folly to use peekBytes() instead.  Eventually I think we
should add a FOLLY_DEPRECATED attribute to peek().  I will wait to add this tag
until after converting a few other projects to use peekBytes(), though.

Reviewed By: alandau

Differential Revision: D3337302

fbshipit-source-id: 14220a059d915bf5adc66b8b283f7228b796a4dc