Remove error-prone methods of BinaryRef.

A user shouldn't care about the internal state, and these methods by
their very nature require asserting a predicate on the internal state.
As such, they cannot be used safely without introducing hidden
long-distance dependencies on the manner of construction of the
BinaryRef.

Use writeAsBinary(raw_ostream &) and writeAsHex(raw_ostream &) if you
need to access the data in a binary or hex format.