Run Miri everywhere

[danielsn]

What does this PR do?

1. Runs Miri in CI on all of the library, not just profiling
2. Explicitly "ignores" the tests where Miri cannot currently run

Motivation

Miri is great for catching bugs, and as much of the library as possible should take advantage of that.

Additional Notes

1. Miri does not run on Tokio tests for now: Don't start the reactor when running Miri tokio-rs/tokio#3360
2. Miri cannot be directly disabled on doc-tests, so I had to use a slightly ugly patch there cargo miri test does not run doc-tests rust-lang/miri#584 (comment)
3. None of the issues I saw appeared to be bugs: they generally related to syscalls that Miri doesn't currently support. We should investigate enabling these tests as Miri advances

How to test the change?

Should just run in the CI

Describe here in detail how the change can be validated.

For Reviewers

• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[morrisonlevi]

Looks good to me, unless some specific team doesn't want to add #[cfg_attr(miri, ignore)] to every new test to some subdirectory.

[ivoanjo]

Looks really good! 👍