Declare a JSON object. The declaration takes the same format as
using in record/1 from library(record). E.g.
?- json_object
point(x:int, y:int, z:int=0).
The type arguments are either types as know to library(error) or
functor names of other JSON objects. The constant any
indicates an untyped argument. If this is a JSON term, it
becomes subject to json_to_prolog/2. I.e., using the type
list(any) causes the conversion to be executed on each element
of the list.
If a field has a default, the default is used if the field is
not specified in the JSON object. Extending the record type
definition, types can be of the form (Type1|Type2). The type
null means that the field may not be present.
Conversion of JSON to Prolog applies if all non-defaulted
arguments can be found in the JSON object. If multiple rules
match, the term with the highest arity gets preference.
