constexpr as much of half as possible

[lgritz]

This is the remaining section of half that can be easily constexpr.

The way they are currently implemented, the basic half-from-float ctr
and half-to-float cast are not able to be constexpr, and that in turn
prevents almost all the arithmetic operations (which convert to float,
do the math, then convert back) from being constexpr.

This is the closest we're going to get with the current implementations.
The most offensive parts to constexpr are the use of table lookups, and
the strange overflow() trying to raise a hardware overflow exception.
It's not hard to imagine alternate implementations that don't work the
same way at all, but we must leave that for another day (and maybe for
somebody who is more willing to chase down ever corner case of IEEE754
logic).

I added private mantissa() and exponent() helper methods, using those
helped me make some other public methods be constexpr in C++11 by
making their body be a single return statement (if you spread over
multiple statements, you become constexpr in C++14 only on some
compilers). We could make those public if people think there are other
uses for these helpers.

Signed-off-by: Larry Gritz [email protected]

[cary-ilm]

LGTM