Move to Uint8Array and js-sha256 for browsers

[junderw]

This will move the whole chain of base-x, bs58, and bs58check to remove Buffer.

The biggest change security-wise is that It adds a dependency to js-sha256.

I have reviewed their source code for 0.9.0 and it looks good to me.

I also tested browserify, and it uses the hash_browser.js and includes the js-sha256 dependency properly.

[gruve-p]

NACK on adding an unmaintained dependency of js-sha256.

unsafe eval issue: https://github.com/emn178/js-sha256/issues/26 https://github.com/near/near-api-js/issues/757#issuecomment-1040240093 (they are even replacing js-sha256: https://github.com/near/near-api-js/pull/800 )

Last commit was in 2017. Author is not responding on issues, nor is he interested in giving somebody else permissions to maintain it: https://github.com/emn178/js-sha256/issues/29#issuecomment-974620795