enhancements vtab_optimizations

Repeated method lookup via the vtab can be redundant. Consider the following code

cdef class Root:

    cdef long i

    def __cinit__(self):
        self.i = 0

    cdef void inc(self):
        self.i += 1

cdef class A(Root):
    def inc_many(self, long N):
        cdef long i = 0
        for i from 0 <= i < N:
            self.inc()

Running A().inc_many(10**9) takes 3.51 seconds on my machine. Assigning the self.vtab to a local pointer, and using that for all the calls to inc cuts down the runtime to 3.39 seconds, and assigning the vtab to a local struct (rather than just a pointer, though this has higher overhead) cuts it down to 3.04 seconds. Given that types of objects are immutable, this might be worth doing in some cases.