core:rt: A few micro-opts
core::rt: Add a test of standalone use of the runtime
core: Use a global lock instead of runtime lock for os::getenv, etc. #4726
core: Replace use of libc::getenv with os::getenv
core::rt: Initialize logging
core::rt: Error handling for TcpStream.read
core::rt: Don't abort when reporting an unknown uv error
core: Turn task::unkillable, etc. into no-ops in newsched. #6377

Not necessary just yet but they make ARC not work.
core::rt: Begin implementing Reader extension methods
core::rt: `read` raises `read_error`
core::rt: Add uv timer bindings
core::rt: Fix scheduling logic for enqueued tasks
rt: Rename sched_key to rt_key

It is more general-purpose than holding scheduler pointers
core::rt: Rename Sched.task_queue to work_queue
core::rt: Rename Task to Coroutine