Remove npm `buffer/` library from agent

[gobengo]

One small step toward https://github.com/dfinity/agent-js/issues/222

Tasks

• [ ] Remove npm buffer/ from @dfinity/agent (leaf of dep tree)

Why?

• Will make for smaller agent.
• Will avoid https://dfinity.slack.com/archives/C01MFF5BJP9/p1614203874086100?thread_ts=1614130668.032700&cid=C01MFF5BJP9

Candidate followups:

• [ ] Remove npm buffer/ from authentication
• [ ] Remove npm buffer/ from /{apps,demos}

[MioQuispe]

+1. updated from 0.8.7 to 0.9.0 and getting Buffer is not defined on a fresh project

[jzxchiang1]

I'm still seeing a dependency on buffer in actor.ts... not sure why it's needed.