fix(json): don't leak when lambdas are used as dumps/loads function

fix(json): don't leak when lambdas are used as dumps/loads function

Cache functions on the code, not on the function identity itself:
different lambdas or local functions are different objects but he
underlying code object is the same.

Avoid caching if the function is a closure because it then becomes a can
of worms (if the argument is not hashable etc). Throw a warning in that
case.

Fix #1108

fix(json): don't leak when lambdas are used as loads function