runtime: lock held checking

[prattmic]

As an extension of @danscales' work on lock ordering, we can extend the runtime to check that functions hold their prerequisite locks when staticlockranking is enabled.

[gopherbot]

Change https://golang.org/cl/248577 mentions this issue: WIP: runtime: add world-stopped assertions

[gopherbot]

Change https://golang.org/cl/245484 mentions this issue: WORK IN PROGRESS: runtime: check held locks with staticlockranking

[gopherbot]

Change https://golang.org/cl/250261 mentions this issue: runtime: add sched.lock assertions

[gopherbot]

Change https://golang.org/cl/250262 mentions this issue: WIP: runtime: add heap lock assertions

[gopherbot]

Change https://golang.org/cl/250263 mentions this issue: WIP: runtime: add channel lock assertions

[gopherbot]

Change https://golang.org/cl/250264 mentions this issue: WIP: runtime: add itab lock assertions

[gopherbot]

Change https://golang.org/cl/266718 mentions this issue: runtime: drop systemstack from lock assertions