Add Stroop <=> XLM conversion utility methods

[janewang]

What problem does your feature solve?

The CLI consistently documents the stroop to XLM conversion, but lacks a built in conversion utility. It is an easy thing to add.

What would you like to see?

Lumen <> Stroop

• toStroop
• fromStroop

[leighmcculloch]

Orienting everything around stroops made sense when all assets on the network shared the same number of decimals as the Lumen, but with Soroban, that consistency disappeared. Can we document what the user stories are that we're supporting and the problem we're solving? It's unclear to me if 'stroop utilities' will fully address the issue for more than just lumens, but the issues of working with multi-unit currencies will apply to other assets too.