Generalized adjacent join generator merge

[tiagovtristao]

Release notes: Adjacent JOIN_GENERATORS with EQUIVALENT path conditions are now merged for optimization.

This pull request is a generalization of https://github.com/facebook/prepack/pull/2623 as suggested by @hermanventer.

Extra:

• Lint issues caused by a recent prettier version update are now fixed;
• flow-bin has been re-added to devDependencies as it was causing scripts that depend on Flow to fail during development.