Rename keys_types.json to recipient_keys.json and generalize the format
to support multiple descriptors and multiple recipient keys:
- "key": a single key-expression string becomes "descriptors": an array
of complete descriptor strings.
- "expected": an x-only key string becomes "expected_keys": an array
containing that key; null remains null.
Convert the nine existing cases by wrapping each key expression in tr(),
except the uncompressed literal uses pk(), since tr() does not accept it.
Each case has one descriptor.
