find-type (library name params)

• library -- datatype library, a keyword symbol
• name -- the type's name, a string
• params -- type parameters, a list of param instances

an instance of data-type, or nil

Look up the type named name in datatype library library.

Additional parameters (knows as restricting facets in XSD) can be passed to specify or restrict the type for the purposes of validp.

Return a type instance for this type and the additional parameters, nil if the type does not exist, or :error if the type exists, but the specified parameters are not valid for that type.