Change HexFloat to work with gcc8. (#2109)

When we want to set a the value of a HexFloat to inf or nan, we
construct the specific bit pattern in an appropriately sized integer.
That integer is copied to a FloatProxy object through a memcpy.  GCC8
complains about the memcpy because it is overwriting a private member of
the class.

The original solution worked well because the template to the HexFloat
could be anything.  However, we only used some instantiation of FloatProxy,
which has a construction from that takes its uint_type, so I decided to use
that constructor instead of the memcpy.  This puts an extra requirement
on the templace for HexFloat, but it will be fine for us.

Part of #1541.