T2893
This diff implements @bitgo/utxo-lib transaction builder. Exact raw txs of previous unit tests are recreated. Some shims are needed due to lack of full support in @bitgo/utxo-lib. However, @bitgo/utxo-lib is better maintained and tested than the current state of ecashjs-lib.
So, this diff should be the starting point for organizing a recommended dev stack.
This diff itself is the last proof of concept -- one that is actually working. It will need to be recast as a stacked diff to explain the various shims and steps that are necessary to get this to work. Also as part of the stack, existing `ecashjs-lib` and its dependencies should be deprecated in favor of @bitgo/utxo-lib