Define mathematical constants in constants.h defined as T_CONST rather
than M_CONST, so we don't have to rely on them existing in whatever
libstdc++ we are using.
Refactor the operator overloading, and give examples for why each one
exists.  Also add unit_operator.h monads which let us do maths with
units inside >> streams.
Change constants to be constexpr floats.
Slight changes to make everything work with fixed-point maths using
libfixmath.
Merge branch 'FixedPoint'