test failures with integers enabled

[radix]

I didn't realize that I didn't have integers enabled when running tests until I tried to write integer-specific tests for the new Saturating support I'm working on. When I tried to run them, I got these failures (with master, with u32 enabled):

    si::acceleration::tests::u32::check_units
    si::area::tests::u32::check_units
    si::force::tests::u32::check_units
    si::frequency::tests::u32::check_units
    si::velocity::test::u32::check_units
    si::volume::tests::u32::check_units
    tests::system_macro::op_assign::u32::sub_assign
    tests::system_macro::u32::sub

It seems that travis isn't running tests with ints enabled either.

[iliekturtles]

Known issue briefly mentioned in the v0.16.0 release notes. All the check_units failures are because conversion factors exceed the underlying storage type limits. Others like the u32::sub_assign and u32::sub failures are because quickcheck is generating test values that have a signed result once the operator is applied.

My plan is to implement PartialEq, Eq, PartialOrd, and Ord and then figure out how to resolve these issues.