Use constants instead of types from typenum to describe units

[prehner]

Two major advantages

• Better readable error messages. The type shows up as something like Quantity<_, SIUnit<3, 2, -1, 0, 0, 0, 0>> rather than the typenum gibberish. Not completely self explanatory, but should be much easier to spot errors.
• METER.powi::<P2>() becomes METER.powi::<2>(), so no need to import the obscure typenum::P2

Disadvantage

• This means having to implement addition and subtraction for the units by hand, which is of course rather trivial, but also a lot of boilerplate. Right now I implemented it only for exponents of base units from -6 to 6, but could be increased rapidly if necessary.

Once const_generics can be used in operators the boilerplate can be simplified a lot without even breaking the code (I think).