add usize/isize to generated trait impls

[cosmicexplorer]

It seems like usize and isize could reasonably be allowed to implement ConstantTime{Greater,Less} via the macros that are already used for other integer types. I assumed it was an oversight that this wasn't already done.

[isislovecruft]

Thanks @cosmicexplorer! This looks fine, but generally in cryptographic code we don't really see usage of integer types of platform-defined sizes, so I am curious what the use-case is for this?

[isislovecruft]

See also previous rationale on #83

[tarcieri]

I've encountered a use case for this: when you are using usize as an index, looping over every item of a n-length collection.

In my particular case, it involves computing a value on an entry at a given index, and then selecting whether to store or discard the result based on whether or not the index matches a specified value.

Here's a real-world example: https://github.com/RustCrypto/crypto-bigint/blob/4aee8ce1e185f9c1156a162ecce6164bf80ab915/src/uint/boxed/bits.rs#L40