Fixes #3531.

Closes #2753.

* Potential fix for GH issue #3203

Gets the specialized type if possible from
NameNode.analyse_as_type

This does introduce a potential new bug:
```
cimport cython

just_float = cython.fused_type(float)

cdef OK1(just_float x):
    return just_float in floating

cdef fail1(just_float x, floating y):
    return just_float in floating

cdef fail2(floating x):
    return floating in floating

def show():
    """
    >>> show()
    True
    True
    True
    True
    """
    print(OK1(1.0))
    print(fail1(1.0, 2.0))
    print(fail1[float, double](1.0, 2.0))
    print(fail2[float](1.0))
```
fail1 and fail2 work before this patch but fail with it. It isn't
clear to me if this should actually be considered a bug. It
works in both versions with `cython.floating`, which possibly
suggests analyse_as_type in AttributeNode should also be changed

* Bring attribute.fused types in line

* Removed try-catch

* Fix and test "type in fused_type" special-case

* Added "analyse_as_specialized_type"

* Fixed cpp_operators (handle type is None)

Make CodeWriter inherit from ExpressionWriter in order to support all kinds of expressions without duplicating code. (GH-3528)

* Fixed indexing temps for non-python objects.
* Moved cleanup into release_temps since the temps survived into the result_code.

* Fixed indexing temps for non-python objects.
* Release another temp in PyMethodCallNode.
  Was causing warnings in generators_py.
* Moved cleanup into release_temps since the temps survived into the result_code.

* Prevent fused dispatcher optional arguments being overwritten

If the 5th argument of the regular functions was optional it'd
overwrite the _fused_sigindex default of the dispatcher, causing
type errors at runtime.

closes https://github.com/cython/cython/issues/3511

Closes https://github.com/cython/cython/issues/3430

Closes https://github.com/cython/cython/issues/3430

This allows things to work like:
    # in pxd file
    from libc.math cimport sin

    # in py file
    if not cython.compiled:
        from math import sin  # previously failed with cython compile
           # error because it was assigning to a cdef name

This seems worthwhile because it makes it easier to write code
that re-assigns cdef names and so works in both modes.

Prevent calling PyObject_GC_Track() in __Pyx_CyFunction_New() when instantiating a fused function before the object fields of the fused function subtype are fully initialised.
See https://bugs.python.org/issue38392
Closes GH-3215.
Supersedes GH-3216.