Like assoc but only assocs when value is truthy.  Copied from plumbing.core so that
schema need not depend on plumbing.

The last schema for a class set by declare-class-schema!, or nil.

Globally set the schema for a class (above and beyond a simple instance? check).
Use with care, i.e., only on classes that you control.  Also note that this
schema only applies to instances of the concrete type passed, i.e.,
(= (class x) klass), not (instance? klass x).

Distinguish a value (must be non-nil) as an error.

A meaningful name for a function that looks like its symbol, if applicable.

What class can we associate the fn schema with? In Clojure use the class of the fn; in
cljs just use the fn itself.

for cljs sake (easier than normalizing imports in macros.clj)

TODO: eventually use built in demunge in latest cljs.

Turn on run-time function validation for functions compiled when
s/compile-fn-validation was true -- has no effect for functions compiled
when it is false.

Provide a descriptive short name for a value.