Add CI

[Jon Gjengset]

Almost certainly what we'll want is Flurry's CI config with the addition of loom:
https://github.com/jonhoo/flurry/blob/master/azure-pipelines.yml

We can probably cut out the cargo deny bits for now.

We should also probably include an adjusted version of codecov.yml. Something like

• https://github.com/jonhoo/flurry/blob/master/codecov.yml
• https://github.com/HdrHistogram/HdrHistogram_rust/blob/1bba4121f74016e02bb6a792cddd9c99d49dbf1c/.codecov.yml

I wonder if MIRI, ASAN, and TSAN will actually let the current tests we have pass 🤔

[Jon Gjengset]

I've set up Azure Pipelines now, so in theory it should just be a matter of adjusting azure-pipelines.yml in a PR now if anyone has the time 😅

Or, even just fix the things that are already broken (most notably that we require nightly).

[Jon Gjengset]

This is now done! Although admittedly I'm tempted to move over to GitHub Actions...