Support singular duration terminology.

[SevereOverfl0w]

(t/new-duration 1 :minute) doesn't work, given the interface, it feels like it should. Minor semantic thing.

[henryw374]

in java.time (and ECMA Temporal) singular terminology is always for fields of a Temporal (date thing), e.g. date.getMinute() and plural is always for fields of a TemporalAmount (period, duration). Tick maintains that, so you can know when to use t/minute vs t/minutes.

I worry this change might cause confusion on something already pretty subtle. feel free to reopen with more discussion.