Discovered [here](https://github.com/numpy/numpy/issues/10528)

Consider the following structure, and a memoryview created from it:

    class foo(ctypes.Structure):
        _fields_ = [('one', ctypes.c_uint8),
                    ('two', ctypes.c_uint32)]
    f = foo()
    mf = memoryview(f)

We'd expect this to insert padding, and it does:

>>> mf.itemsize
8

But that padding doesn't show up in the format string:

>>> mf.format
'T{<B:one:<I:two:}'

That format string describes the _packed_ version of the struct, with the `two` field starting at offset 1, based on the `struct` documentation on how `<` should be interpreted:

> No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’

But ctypes doesn't even get it right for packed structs:


    class foop(ctypes.Structure):
        _fields_ = [('one', ctypes.c_uint8),
                    ('two', ctypes.c_uint32)]
        _pack_ = 1
    f = foo()
    mf = memoryview(f)

The size is what we'd expect:

>>> mf.itemsize
5

But the format is garbage:

>>> mf.format
'B'  # sizeof(byte) == 5!?