json: Support typeof in set and map types

json: Support typeof in set and map types

Implement this as a special "type" property value which is an object
with sole property "typeof". The latter's value is the JSON
representation of the expression in set->key, so for concatenated
typeofs it is a concat expression.

All this is a bit clumsy right now but it works and it should be
possible to tear it down a bit for more user-friendliness in a
compatible way by either replacing the concat expression by the array it
contains or even the whole "typeof" object - the parser would just
assume any object (or objects in an array) in the "type" property value
are expressions to extract a type from.

Signed-off-by: Phil Sutter <phil@nwl.cc>