Audit for all usage of unchecked arithmetic and update nomenclature

[cwgoes]

Looking through the codebase for unchecked arithmetic I still find quite a bit, e.g.:

• here
• here
• many more, just searching...

We should:

• Remove ALL support for unchecked arithmetic. We should never use it.
• Update all arithmetic to use checked_sub, checked_add etc. names (don't overload + / -) to make it clear that we're using checked arithmetic (and so a search for only + or -, for example, should return only internal implementation code instead decimal types and no usage other than that).