Add paddedDecimal, for zero-padding.

[quasicomputational]

This doesn't currently specialise as well as decimal, but it should be easy to give it an analogous structure if that would help. In the absence of benchmarks, I've left it in the simpler form.

[quasicomputational]

Also, I've speculatively put this as being since 1.2.4. If events happen on the way to getting this merged and released, that may need to be updated to keep it from being a lie.

[quasicomputational]

Ping? Is there anything I need to do before this is ready to merge?