Changes include:

• Add (f Float16) Bits() uint16.
• Update docs and unit test to include the new method.

Bits returns the IEEE 754 binary16 representation of f, with the sign bit of f and the result in the same bit position. Bits(Frombits(x)) == x.

Calling Bits should inline as a simple type cast so it doesn't add bloat while making the API nicer.