Drop outdated Rusha and BigInteger
Background
The Rusha library was added as part of issue 4465.
The library has been updated upstream already a few times, but turns out we actually don't need it at all, thanks to the standard Web Crypto API.
What to change
Use the Web Crypto API instead of outdated 3rd party libraries, provide a fallback for older testing environments, so that CI can still catch possible regressions now, or in the future.
Integration Notes
The exported verifySignature
utility is now asynchronous so that every consumer of such utility must handle its asynchronous nature. The signature is identical to the one we had before.
Edited by Andrea Giammarchi