Improve encapsulation for source location calculation in `IonSlice` and `IonStream`

[desaikd]

This branch is not used for IonSlice, right? That suggests to me that things aren't encapsulated the right way. It doesn't necessarily need to be addressed now, but this could easily become a friction point if we're trying to refactor things at some point in the future (either related to source location or related to IoBufferHandle, etc).

Originally posted by @popematt in https://github.com/amazon-ion/ion-rust/pull/941#discussion_r2017268891