Fix std.mem.orderZ for types for which 0 is not the least element

[Fri3dNstuff]

This PR fixes an oversight in std.mem.orderZ that caused the function to sometimes produce wrong results for slices of signed integers and floats.

Added test case:

try testing.expect(orderZ(i8, &.{ -1, -2 }, &.{ -1, -2, -3 }) == .lt);

[Fri3dNstuff]

There's still the issue of floating point numbers to be discussed: IEEE-754 (in its infinite wisdom...) declares 0.0 == -0.0, meaning the following will return .eq:

std.mem.orderZ(f32, &.{ 1.0, 2.0 }, &.{ 1.0, 2.0, -0.0 })

Should this be addressed? If so, other functions operating on sentinel-terminated values (std.mem.len, etc) ought to be looked at too.