Remove unnecessary dependency on 2's complement

Remove unnecessary dependency on 2's complement

Although the bit-representation of signed integer types in C99 is
implementation-defined and can be sign-magnitude, one's complement, or
two's complement[0], the conversion of negative values to an unsigned
integer type is defined to be adding 1 plus the maximum value of the
unsigned type[1].

Since -1 + 0xffffffff + 1 == 0xffffffff, just using u32 here has the
right behavior without relying on the representation of signed integers.

[0] http://port70.net/~nsz/c/c99/n1256.html#6.2.6.2p2
[1] http://port70.net/~nsz/c/c99/n1256.html#6.3.1.3p2