specs: need new generic-function-method spec type

[cgay]

In the System test suite the spec says:

  open generic-function \= (<date>, <date>) => (<boolean>);

The failure says:

      date-protocol-functions-test failed
        function = argument 0 type {class <object>} is a supertype of the specified
            type {class <date>} failed [expression "subtype?(actual, spec)" evaluates
            to #f, not a true value.]

(The sense of sub/super-type in the error is incorrect but that's not the issue here.)

This is checking the wrong thing. What this spec wants to check is that there is a specific method on = for (<date>, <date>) but what it's actually checking is that the = gf exists and is defined on parameters that are subtypes of <date>.

I propose a new spec:

  generic-function-method \= (<date>, <date>) => (<boolean>);

to do the needful here.

[cgay]

In #112 I remove generic-function specs because generic function does exactly the same thing. That works for define function, define generic, and for implicit generic functions defined by define method (not that that's much use).

I think the spec for checking that a particular method exists should just be called method, assuming that doesn't cause any parsing problems.