Prepare for 4.0.0

[briancavalier]

Let's collect a list of what it would take to a release 4.0.0, without any preconceived notions of it having to be a "big feature release" or total rewrite, like 3.0.0 was. There's still enough flexibility in the current codebase.

Here are a few to start. These are all breaking changes that require a major semver bump:

• Remove deprecated methods across the board
• Either move unfold and iterate to a separate npm package or undeprecate them. People have told us they find them to be useful.
• Make ES6 Promise shim leave global Promise intact if present.
• Remove progress

[FuriouZz]

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