wip
core::rt: Just some poking at the I/O docs
rt: Add rust_dbg_next_port for generating test port numbers
core::rt: Add a test mod and put run_in_newsched_task there
core::rt: Use generated port numbers in tests
core::rt: Unignore some networking tests

These should work now, I hope
core::rt: Remove redundant copy of run_in_newsched_task
core::rt: Don't directly create scheduler types in I/O tests

There are some better abstractions for this now
core::rt: Fix a broken uvio test
core::rt: Add `next_test_ip4` for generating test addresses
core::rt: Improve docs for run_in_newsched_task testing function
core::rt: Listener constructors are called  and return a
core: Ignore two long-running tests
core: Speed up a test case
core::rt:: Implement Reader/Writer for MemReader/MemWriter
core::rt: Add LocalServices for thread-local language services

Things like the GC heap and unwinding are desirable everywhere the language
might be used, not just in tasks. All Rust code should have access to
LocalServices.
rt: Don't make memory_region depend on rust_env

I am going to use memory_region and boxed_region as the local heap
in the new scheduler, for now at least, and I don't have a rust_env
available.
core::rt: Add the local heap to newsched tasks

Reusing the existing boxed_region implementation from the runtime
core::rt Wire up task-local storage to the new scheduler
core::rt: Make I/O constructors return Option instead of Result

For consistency, for all I/O calls, inspecting the error can be done with
the io_error condition.