replace bignumber.js

[dynst]

The author of bignumber.js also wrote the much smaller, lighter big.js as an alternative; would it be good enough as a replacement?

Or could another package like fraction.js, or even just a very thin wrapper around the native BigInt be up to the job of representing BaseAmount and AssetAmount, actually?

Other packages to consider:

• typescript-bignumber
• bigint-money
• @cosmjs/math