mocker: `_FooBehaviour[T]` should be exported

[youta-t]

Values returned from New...Call().ThenReturn(...) can not be set as test fixture.

We cannot do this:

type Case struct {  ... }

for name, tt := range map[string, Case]{
    "test case": {
        foo: gen_mocker.NewFCall(...).ThenReturn(...),
    },
} {
    t.Run(name, func(t *testing.T) { ... })
}

Since _[T]Behaviour, return type of .ThenReturn is not exported, we cannot declare a field typed _[T]Behaviour in struct Case.