Add sync primitives

[vakaras]

This is a follow up PR for #1284 that adds support for the missing synchronization primitives.

Sorry for flooding with PRs, but my internship is coming to an end and I need to get things out.

Fixes #1419

[RalfJung]

Looking good, all nits resolved. :) Thanks a lot!

Please rebase, and squash the niting in the end to a few commits. (No need to have a beautiful history but 66 commits is a bit much.)

[vakaras]

It seems that 200ms is a too small range for a timeout.

[RalfJung]

Looking great, thank you so much. :)
@bors r+

[bors]

📌 Commit 34ddd77 has been approved by RalfJung

[bors]

⌛ Testing commit 34ddd77 with merge bfe030d...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: RalfJung
Pushing bfe030d to master...

[jonhoo]

Oh boy oh boy oh boy! After you've merged all your changes @vakaras, any chance you could do a little write-up to demonstrate what now works in miri?

[RalfJung]

I plan to write a blog post once my thesis is finally submitted (~2 weeks), mentioning all the things that landed in the last 6 months or so (panics, file system access, cross-target-execution, concurrency... there's probably more and there are some minor but notable things I still plan to do before the post) but of course I'd happily link to a more detailed write-up about the concurrency parts. :D

[vakaras]

Oh boy oh boy oh boy! After you've merged all your changes @vakaras, any chance you could do a little write-up to demonstrate what now works in miri?

Not earlier than in the next three weeks because I need to get Prusti working again as soon as possible.