Add i128 support

[macisamuele]

This fixes #487.

This PR adds support to 128-bits integers.
To avoid the introduction of a lot of boilerplate and code duplication I've modified the number::N to hold 128-bits integer.

I've not verified the whole library yet to ensure that all the integers flows are covered.
I opened this to have an initial feedback about the process.

[dtolnay]

Thanks! I am closing the PR because I am not interested in changing the representation of Number or raising the minimum required Rust version. If a higher fixed precision Number type is required, I would recommend providing that in a separate library.

[rslife]

Hello @dtolnay.

Now that almost a year has passed, would you be open to re-considering raising the minimum Rust version required?

Related issue: pyfisch/cbor#140