Replace uses of `Buffer` with `b4a`

[mafintosh]

@kasperisager loving this, wondering how we could test this... maybe just manually bundling?

[kasperisager]

I ran a bundled version in a browser and encountered the following issues:

• Hypercore extends EventEmitter.
• sodium-javascript references global. It should be globalThis.
• hypercore-crypto also uses Buffer.
• sodium-javascript lacks crypto_secretstream_xchacha20poly and also uses Buffer.
• streamx uses EventEmitter.
• noise-curve-ed uses sodium-native, not sodium-universal.
• noise-handshake uses bint8array which uses string_decoder which uses safe-buffer which also uses Buffer.
• @hyperswarm/secret-stream also uses Buffer.
• random-access-memory uses process.nextTick and also uses Buffer.
• random-access-storage uses process.nextTick and EventEmitter.

With all of those patched in my local dependency tree, I can construct a Hypercore in the browser, though it doesn't correctly open yet.

[mafintosh]

Nice, good to track.

[mafintosh]

Looks great, merging.