Relaxed some of the checks of fused functions to be consistent with general CyFunctions.

```
 # cython: binding=True
def f(arg):
  pass

class C:  # or cdef class...
  f = f
  def g(self, ...):
    pass

C.f(something) or C().f() # doesn't enforce any checks on the type of arg -
 # with a fused function it does.

C.g(something) # assumes that self is "C" (at least for a cdef class)
 # but doesn't check it. A fused function enforces that it is C.

C.f() # fails with a fused function claiming too few arguments, even though
 # default arguments may make it a valid call
```

Obviously removing checks does make things a little less safe, but it
is consistent with the more general function behaviour. (I'm doing
this as part of a broad plan to abuse fused functions to be a bit
cleverer about decorators, but I don't think the motivation hugely
matters for this change)