FloatingPoint: Clean up and document header, format. (#5453)

FloatingPoint: Clean up and document header, format. (#5453)

This cleans up the src/util/floatingpoint.h.in header to conform to the
code style guidelines of CVC4. This further attempts to fully document
the header. The main changes (except for added documentation) are
renames of member variables, getting rid of the redundant functions
FloatingPointSize::exponent() and FloatingPointSize::significand(),
and a more explicit naming of CVC4 wrapper types vs symFPU trait
types. Else, it's mainly reordering and formatting.