Fix unsigned error value casting

[jagerman]

When casting to an unsigned type from a python 2 int, we currently cast using (unsigned long long) PyLong_AsUnsignedLong(src.ptr()). If the Python cast fails, it returns (unsigned long) -1, but then we
cast this to unsigned long long, which means we get 4294967295, but because that isn't equal to (unsigned long long) -1, we don't detect the failure.

This commit moves the unsigned casting into a detail::as_unsigned function which, upon error, casts -1 to the final type, and otherwise casts the return value to the final type to avoid the problematic double cast when an error occurs.

The error most commonly shows up under Python 2 wherever long is 32-bits (e.g. under both 32- and 64-bit Windows, and under 32-bit linux) when passing a negative value to a bound function taking an unsigned long. The same issue would show up, however, for other size-increasing casts regardless of the Python version (for example, when binding a function taking gcc's __uint128_t type).

Fixes #929.

[jagerman]

The added tests segfaulted under PyPy; it turns out to be a latent issue (i.e. occurs in current master) when attempting to call:

m.def("f", [](int32_t) {});

with a value that fits in a long but not a int32_t (e.g. 3333333333 on a 64-bit-long architecture).

I've squashed the fix into this commit: we aren't allowed to call PyErr_ExceptionMatches() if we didn't actually get an exception from the conversion. (We were calling it unconditionally if either an exception happened, or we detect an integer overflow case).

[dean0x7d]

This looks great. 👍 for adding the detailed tests and catching the exception issue.

[dean0x7d]

Can you add a comment above this line to associate it with the Python test function:

// test_integer_casting

[jagerman]

Done.