citeproc-js date input format change

Looking at the input format used for dates in citeproc-js, I decided
that it is excessively awkward to validate and to process, and too
much prone to programming error. I’m planning to adapt the code,
tests and documentation to a new, simpler format.

This is a lossless conversion; all that will change is the JSON
structure used to feed existing date elements to the processor. A
description of the new format is here, for anyone with an interest in
this change.

http://gsl-nagoya-u.net/http/pub/citeproc-doc.html#dates

Unless there are strong objections, I’ll push this into the code base
in the next day or so.

(At the same time, I plan to refactor the human-readable format of
names in the tests to reflect what the processor actually sees. The
"convenience" shorthand I’ve used in the human-written, human-readable
test files can be misleading. It has already cost at least one
developer some extra time, which is a clear signal that it should go.)

Frank