Proposal: Deadlock detection mode for the runtime

[DemiMarie]

This is a proposal for a deadlock detection mode for the runtime. In this mode, when the runtime detects that a goroutine is unreachable, it prints the goroutine's stack trace to stderr and then panics the goroutine. If the panic is uncaught, the runtime will log the stack trace to stderr, but not kill the program (this is different from the normal case).

[minux]

Possibly dup of #13759?

[rsc]

Duplicate of #13759 (idea accepted but no clear implementation approach).