Summary

Yarn 0.27 has workspaces feature that should be a better alternative to Lerna boostrapping.
Yarn workspaces still don't replace Lerna (and probably won't do it completely for a long time), so Lerna publishing is still used for Jest.

This PR replaces Lerna bootstrap phase with Yarn workspaces.

Things to discuss:

1. Unlike Lerna Yarn hoists workspaces to the root node_modules.

It means that integration/examples test don't run in isolation from the workspace root so I removed symlinking to babel-jest et al. at the tests boostrapping.
One test integration_tests/__tests__/transform.test.js required to be running in isolation, so I changed the test bootstrap to execute on a copy on a temp folder.

2. As of Yarn 0.27.2 workspaces are still fresh, I am working on making it snappy.
   Known issues:

• changes in workspaces (non root) package.json are not automatically invalidating integrity check
• logs may have confusing lines when working with workspaces
• yarn add/upgrade/remove is not fully tested from within workspaces

3. PR to support Yarn workspaces as first class citizen in Lerna Support Yarn workspaces to replace bootstrap command lerna/lerna#899

Test plan

• yarn test-ci
• lint/flow/build tasks work per workspaces correctly
• ran yarn run publish and lerna seems to handle version bumping and npm publishing correctly