Lab Refactor: Tier 3: Fix extrapolateFromXdr

[piyalbasu]

extrapolateFromXdr doesn't work in node seemingly because node handles Buffers differently than the browser. This makes it impossible to test properly in @testing-library/react.

This is also true of actions/xdrViewer/fetchSigners. In Node, Buffers don't have an equals method, which throws an error.

Refactor theseto work across both engines.