technically `literal` and `year` keys are in CSL-JSON
or at least they are now

- updated serde because I thought it would fix an issue,
  may as well keep it

all this was done through the new crates/io/tests/json
test suite.

test:
- parses the JSON schema to auto-generate test cases
- tests for unknown variables
- tests all classes of variable are parsed as the correct
  AnyVariable variant
- tests that these tests are exhaustive

test-suite:
- adds date_IsUncertainDate as a pure boolean test

dates:
- also fixes <if is-uncertain-date="issued"> not being implemented
  (it passed date_Uncertain spuriously, because it rendered circa term anyway)
- ignores unrecognised fields in dates
- ignores additional entries in date-parts arrays
- ignores more invalid values
- accepts bool for circa
- stores circa on literal as well
- log warnings for many cases

variables:
- adds _everything_ from the latest CSL-JSON schema