bpo-10746: ctypes: Fix PEP 3118 type codes for c_long, c_bool, c_int - #31

Ctypes currently produces wrong pep3118 type codes for several types.
E.g. memoryview(ctypes.c_long()).format gives "<l" on 64-bit platforms,
but it should be "<q" instead for sizeof(c_long) == 8

The problem is that the '<>' endian specification in the struct syntax
also turns on the "standard size" mode, which makes type characters have
a platform-independent meaning, which does not match with the codes used
internally in ctypes. The struct module format syntax also does not
allow specifying native-size non-native-endian items.

This commit adds a converter function that maps the internal ctypes
codes to appropriate struct module standard-size codes in the pep3118
format strings. The tests are modified to check for this.

Example of the current problem in practice:

>>> import numpy, ctypes
>>> numpy.asarray(memoryview(ctypes.c_long(42)))
/usr/lib64/python3.5/site-packages/numpy/core/numeric.py:482: RuntimeWarning: Item size computed from the PEP 3118 buffer format string does not match the actual item size.
  return array(a, dtype, copy=False, order=order)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python3.5/site-packages/numpy/core/numeric.py", line 482, in asarray
    return array(a, dtype, copy=False, order=order)
ValueError: setting an array element with a sequence.

• struct format string syntax
• bpo-10746

https://bugs.python.org/issue10746