std::rl is unsafe

[brson]

linenoise uses global state without mutexes. My preferred fix is #3915. For now I will mark the interface unsafe.

[brson]

Also, the completion callbacks need to be cleaned up at some point. This API should probably be attached to some ReadlineContext that tries to pretend like it's not a global API, cleans up the Rust pointers on destruction, and tries to be defensive about multiple clients trying to use the API concurrently.

[graydon]

blocked on #553 most likely

[emberian]

#553 and #3915 are both fixed with static mut.

[huonw]

Triage: hopefully fixed when #9096 lands.