_helpers: fix -Wsign-compare warnings

Python 3 adds -Wsign-compare to the compiler arguments.  Address the
issues that this causes gcc to warn about.  Disable the warnings for
the INTEGRAL_ASSIGNMENT_FITS macro since it's intentionally doing
something that the compiler can't tell is OK (assuming it is), add
some simple tests, and change the types and code in other places to
avoid the suspect comparisons.  Currently clang doesn't produce the
warnings.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>