This significatnly refactors the amount formatting code to make
formatting more configurable. The main addition is the
`amount::Display` type which is a builder that can configure
denomination or other things (possibly more in the future).
Further, this makes all representations of numbers minimal by default,
so should be documented as a possibly-breaking change.
Because of the effort to support all other `fmt::Formatter` options this
required practically complete rewrite of `fmt_satoshi_in`. As a
byproduct I took the opportunity of removing one allocation from there.
Closes#709
4f1200d629