Buffer being clobbered in capacitor and other non-node environments

[schancel]

https://github.com/indutny/bn.js/blob/master/lib/bn.js#L51-L55

Buffer will already be defined, and require() simply returns an empty object in Capacitor. This results in Buffer being reset to undefined and triggering asserts later on. Not sure what the best course of action is here due to variable lifetimes.

Maybe:

const tryPolyfillBuffer = () => {
  if(typeof Buffer !== 'function') {
    try {
      return require('buffer').Buffer;
    } catch (err) {
      return Buffer;
    }
  }
  return Buffer;
}

let Buffer = tryPolyfillBuffer();

[fanatid]

Closed in #260