JBIND/JMATCH: better optional value syntax (default values? nested destructuring?)

[aartaka]

Right now, jbind/jmatch destructuring patterns can contain the (var &optional var-p) list to denote an optional object property or array element. However, this syntax doesn't allow providing default value for the case this optional value is not provided. CL-native optional arguments allow that, and it's useful.

So the syntax should be extended to

(var &optional default-value var-p)

It's not backwards-compatible, so it can only happen on the next major release.

[aartaka]

Alternatively, use the second value as the nested destructuring pattern, so that one can destructure inside the object when it's present, and get NIL is it's not. Seems pretty intuitive for a destructuring macro?