* changed BigNumber to a class that extends Decimal.clone(...)
* it works... if you patch decimal.js
* added a test that checks if sum of Decimals works
* minor improvement of code quality
* updated Decimal.js to 10.3
* removed the optional chaining operator to keep compat w/ Node 12
* removed try/catch from isBigNumber
Co-authored-by: Jos de Jong <wjosdejong@gmail.com>