Add special cases for %da and %ta to +slaw
Add special cases for %da and %ta to +slaw

+slav calls +slaw with %da and %ta in several places across multiple
vanes. This refactors the date parsing code into its own arm so it
can be called from +slaw.

jfranklin9000/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,joemfb/arvo,urbit/urbit,urbit/urbit,urbit/urbit,jfranklin9000/urbit,belisarius222/arvo,joemfb/arvo,ngzax/urbit,belisarius222/arvo,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit