Support `Decimal` in unit catalogue

[yunruse]

Per #18 it would be nice if all units were Decimals. This is a "would-be-nice" feature: float accuracy is more than good enough for almost all purposes.

To do this, I think the following would be nice:

• Make some FlakyDecimal(Decimal) that is interoperable with float
• #x
• Make all units FlakyDecimal by default

[yunruse]

Of course, users could use some config flag to just make things float if this becomes a performance annoyance. (To be honest I haven't really been thinking about performance...)