Allow any type, including predefined ones, to become a base unit

[erikerlandson]

The coefficient of a base unit is always 1. The only thing I need for a base unit is (1) a way to detect that it is a base unit, and (2) long and abbreviated name.

Allowing any type to serve as a base unit might be very powerful - associating quantities with any kind of object.

Directly declaring an implicit UnitDecl is one possible option, or using a special variation to detect when something is declared as a BaseUnit directly, or if it is being applied to an existing type

[erikerlandson]

I have implemented this (opt-in policy) on #66