Make vtable order of extension types with fused methods only dependant on the original declaration order (e.g. in the .pxd file).
Previously, fused methods were specialised and expanded on first use, which lead to an arbitrary order in the vtable.
Also fixes compile failures when inheriting from base types with fused cdef methods.
Fixes #1873.

PyPy 3 fixes

Note that on pypy, SOABI tags start with '.pypy' which contains
the string '.py'...

It is used even when CYTHON_USE_TYPE_SLOTS is disabled.

Disable it on pypy3, since it doesn't support PEP 442 yet.

Remove special case support for block name suffix after "XYZ.proto" utility code title. Replaced by existing "@proto_block" option.

Refactor coercion predicate function to make it clearer what happens for the template based conversions and for the non-template string conversions.

Avoid deprecation warning for "Py_UNICODE_strlen()" by always using the version that we ship anyway.

Improve C macro guard: __Pyx_GetAttr() should be optimised exactly iff __Pyx_PyObject_GetAttrStr() is also optimised.

Avoid inlining some generic fallback functions. However short they are, they are just fallbacks and shouldn't get in the way.

Refactor dependencies between buffer/memoryview utility code to avoid unused C code and remove explicit utility code dependency lists from Python code where possible since they are much better expressed right in the C code.

Split 2-tuple unpacking helper function into a (still) inlineable fast-path and a slow fallback function that does not need inlining.

Also avoid double-check overhead for strigns and string subtypes in Py2: only needed in Py2.6 and not really worth it.

Supply a general macro for fast exception setting without value (e.g. StopIteration).

Mark intended fall-through steps in switch statement of int-pow() implementation with "CYTHON_FALLTHROUGH" to avoid compiler warnings.