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.

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.)