Mathematical and convenience operations on the numeric types (Int, Decimal, Float)

[jobarr-amzn]

We need more mathematical and convenience operations on the numeric types, e.g. +, -, *, /, % and various type conversions like to_f64 even if lossy.

For e.g. Int these are quite easy to implement, for Decimal it's more complicated. These conveniences make the Ion values much easier to work with.