Refactor: Move core functionality to index.js

[aphelionz]

This PR starts us off towards browser support by migrating core functionality from cli.js to index.js. It's a WIP still but I think it's good enough to take out of draft. See the "What's left" section below for a start of what should be done before approval.

What it does:

• Adds a single benchmark to the benchmarks folder in the repo that just calles new Date()
• Moves start, runOne and getElapsed to index.js
• Alters the signature of start but then applies that signature at the call site in cli.js so that nothing should functionally change
• Wraps the cli.js in if (require.main == module) so that simply requiring it doesn't run the benchmarks automatically
• Moves a lot of the previously-global constant declarations inside the start function
• Adds a number of tests to test/cli.test.js that run the start function

This gives us near 100% test coverage 🎉 and also paves the way for building with webpack et al and doing some browser benchmarking

These PRs should be merged first!

• Making more general to OrbitDB #18
  • Remove rimraf for now and handle output in a separate PR.
• Refactor/src and test #19
  • Move ./benchmarks to ./test/benchmarks or something to get it out of the root repo
  • Move cli.js and index.js to a /src folder
• Moving global.gc to index.js #21
  • Move gc to global.gc and remove the need for --expose-gc

What's left:

• Actually capture and check the outputs of the new tests to ensure they're actually doing what they're supposed to be doing instead of just calling await start and being happy that there were no errors
• Put output back but perhaps consider console.log or something so there's nothing node.js-specific in index.js (should all be captured in cli.js and perhaps an upcoming browser.js)

(Above moved to new issues)

Fixes #15