Predicate validation in JBIND

[aartaka]

jbind is quite useful already, but it is somewhat hard-coded in functionality. Allowing arbitrary predicates is probably the biggest customization that can happen to it, so that one can do:

(njson:jbind #(1 #'evenp 3 (lambda (n) (= n 4)) five)
    five)

And make sure that an array like #(1 8 3 4 X ...) passes the check and returns the provided X.

Problematic moment is: function and lambda forms conflict with optional value parsing due to being symbol conses.

• So it's unclear what (function foo) means: a predicate reference or optional binding for function variable.
• lambda is easier to parse and harder to miss due to arglist presence, but JBIND/JMATCH: better optional value syntax (default values? nested destructuring?) #8 can change that too.
• quoted symbols don't work for predicates either: they are symbol-lead conses too.

So it's either hard-coding the rules for function, lambda, quote etc. (which is really unreliable), or coming up with a new syntax. Risking too much bloat here...