ns spec breaks gen-class forms that use strings as class names

The following valid `ns` gen-class form is reported as invalid by spec:

This is because the ns spec assumes that all class names can be represented by simple-symbols, while in reality some can only be represented by strings.

Approach: Pull out spec for class identifiers which can be either a simple-symbol (class name) or a string and use that in signature (which is used for both gen-class constructors and methods.

Patch: clj-2012-2.patch