strided intervals: min_int() returns a negative value

[lwerdna]

In strided intervals, the maximum and minimum values for a given width are returned by four functions, depending on how you want to interpret the bits.

For unsigned interpretation there are min_int() and max_int().
For signed interpretation there are signed_min_int() and signed_max_int().

Here they are all tested for 32-bit width:

StridedInterval.min_int(32)         # -0x80000000    <-- ERROR
StridedInterval.max_int(32)         #  0xffffffff    <-- OK
StridedInterval.signed_max_int(32)  #  0x7fffffff    <-- OK
StridedInterval.signed_min_int(32)  # -0x80000000    <-- OK

Observed behavior: The unsigned version for 32-bit width returns that the minimum value is -134217728 or -0x80000000.

Expected behavior: It should return 0, the truly minimum value in unsigned interpretation.