Create hyper-util crate

[seanmonstar]

Decide whether to make it a repo or a subdirectory. To hold all the hyper-util goodies.

[seanmonstar]

The two options here are: make a separate repository, or convert the current one to be a multi-crate repository.

Separate Repository

• Separate git tags: The biggest pro is that hyper's repository tags can continue to just be hyper versions, like v0.14.10, v1.1.0, etc, and not have to be shared weirdly like hyper-util-0.3.2.
• hyper-util CI doesn't block hyper releases. While it would be useful to check hyper-util with each commit of hyper, the converse is annoying. We don't want some utility to have broken to prevent being able to deploy a patch release of hyper.

Same repository

• Making changes in unison is easier. If a utility needs an API in hyper, making those changes in the same commit is easier. Separate repositories means it needs to wait slightly.
• Keeping existing file history. It's much easier to keep the git history for files that are being moving from hyper to hyper-util if they stay in the same repo. If there's a simple way to transfer history to a new repo, this pro is less enticing.

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I'm personally more inclined to have a separate repo, for the reasons listed.