Feature request: new package support/i128 to manage 128-bit math

[Shaptic]

What problem does your feature solve?

Converting to/from integer types can be a huge source of bugs, and the use of xdr.Int128Parts will be prolific in the Soroban world as the de-facto way to represent amounts.

What would you like to see?

A new package, support/i128, that deals with everything 128-bit related. This (non-exhaustive) list includes:

• doing math on the value
• correctly representing negative values in the unsigned "halves" of the XDR struct
• safely converting to/from int64 values which represent
• conversions to/from strings with varying degrees of decimal precision/representation

It's likely that there are libraries out there we can leverage or at least borrow code from, but it will still need to be custom to some degree to adapt specifically to the xdr.Int128Parts structure.

What alternatives are there?

Chug along until it becomes a bigger problem.