This makes it possible to refer to more specific parameter types when
they're known. It also means that it doesn't always just reduce to
"any".

This makes it possible to add exceptions to disable errors, and to
configure their severity, using the same syntax as is used for lint
rules.

These properties were being configured with the value `[]`, which we
newly interpret as "unset" rather than "set to the empty list". This
distinction doesn't matter in general, but it does mean that the rules
won't be flagged as deprecated.

This reverts commit bf7fe0b.