Define a message for a given spec or a predicate.
Arguments:
- either a keyword (spec) or a qualified symbol (for a predicate);
- either a string or 1-arg function that takes a problem map and returns a message.

Define multiple messages at once.
Takes a map of sym/keyword => message/function values.

Prints the formatted output of explain-data.

Like s/explain-data: takes a spec and a value
and returns a map with the problem list. Each problem
has clear message found by either a predicate or a spec.

Like explain, but returns the captured output as a string.

Undefine a message for the given sym/kw.

Undefine all the known messages.