Using eosjs-ecc, should migrate to eosjs v21 methodology

[bradlhart]

With the near release of eosjs v21, a push from eosjs-ecc to elliptic has been decided. This repository utilizes eosjs-ecc and should migrate to the new methodology using eosjs v21 and elliptic.

Before the release of eosjs v21, the eosjs edge release on npm will have the necessary functionality as well as the elliptic npm package to create a branch for this repo ready to release after eosjs v21.

Please see related PRs for example migrations:
EOSIO/eosio-reference-chrome-extension-authenticator-app#37
EOSIO/ual-scatter#56
EOSIO/eosjs-ledger-signature-provider#32