Implement Duration normalization - Part 1

[nekevss]

This draft PR is meant to eventually address #19

Currently, this PR is small baseline changes focused on NormativeTime records and balance methods (EDIT: it also adds CalendarDateAdd functionality for the ISO8601). The changes overall are pretty widespread across large sections of Duration that could really use more tests, so the plan is to come back to this once it can be tested a bit more or split it into a couple of PRs.

I'm going to switch this into a normal PR from a draft. After looking through it, #19 will still need to be implemented in a couple PRs, but I think it makes more sense to break the overall update down into smaller chunks

[jedel1043]

I'm seeing the spec corrections make it possible to do the computations using 128bit integers. Will we transition to that representation in the future? (32 bit components + 128 bit normalized repr)

[nekevss]

Yeah, I think it did.

To be frank, I haven't put too much thought into it as of yet. Mostly because I would really like to have a lot more of the spec implemented and some more tests in place to mitigate any edge cases or oddities that may arise off the switch.

Since f128 isn't implemented yet in Rust maybe that should be prioritized after the normalization updates.

[jedel1043]

Pretty straightforward changes, mostly because it's closely following the spec. I just have a single suggestion that doesn't block merging this.