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:
emn178/js-sha256#26
near/near-api-js#757 (comment) (they are even replacing js-sha256: near/near-api-js#800 )

Last commit was in 2017. Author is not responding on issues, nor is he interested in giving somebody else permissions to maintain it: emn178/js-sha256#29 (comment)