elliptic .toBuffer() not browser compatible

[bradlhart]

Version of EOSJS
v21-rc releases and edge release

Describe the bug
bn.js has a method .toBuffer that does not work in browsers. It is not possible to convert private keys from elliptic as a result.

To Reproduce
Steps to reproduce the behavior:

1. Convert an Elliptic Private Key to eosjs standard

Expected behavior
The private key is converted correctly.

Desktop (please complete the following information):

• OS: macOS
• Browser chrome
• Version edge

[bradlhart]

Currently in edge version and will be included in v21 stable release.