Add labels for integer and rational magnitudes

[chiphogg]

Think of this as "just enough of #85 to unblock #105". The fact that
our common unit labels don't tell you the size of the unit (#105) has
really, really been bugging me for a long time. Recently, I realized we
don't need to do all of #85 to get it! Instead, all we need to do is:

1. Build a mechanism that we can easily extend.

2. Cover the most important use cases.

This PR creates the MagnitudeLabel trait mechanism (also accessible
via a function/value interface as mag_label). We enumerate the
various categories of magnitudes that we can label, defaulting to
"unsupported". The first two supported categories are integers (that
fit in std::uintmax_t), and rationals.

We also add a trait, has_exposed_slash, looking forward to the obvious
use case of auto-generating labels for scaled units. Those labels will
have the form "[M U]" for a unit of label "U" scaled by a magnitude
of label "M". If has_exposed_slash is true for a given magnitude
label, then we'll know to make this "[(M) U]" instead.

Finally, we move a couple of StringConstant-ish utilities into
"string_constant.hh", so that we can use them in our implementation.

Helps #85.

[geoffviola]

Implementation, tests and documentation looks good. Thanks!