valgrind errors around stacks and context switches in newsched

[brson]

I think these are probably because we're not calling VALGRIND_STACK_REGISTER, but sometimes I see many instances of this type of error:

==936== Invalid read of size 8
==936==    at 0xAA8598: rt::uv::uvio::__extensions__::write::anon::expr_fn_40723 (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA72E1F: rt::sched::__extensions__::meth_39073::run_cleanup_job::_d89672ea64dda16::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA6F383: rt::sched::__extensions__::meth_38960::resume_task_immediately::_a2575627634c4428::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA9A0CB: rt::uv::uvio::__extensions__::tcp_connect::anon::anon::expr_fn_40192 (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xABC2AB: rt::uv::net::__extensions__::connect_cb::__rust_abi (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xABC49C: rt::uv::net::__extensions__::connect_cb::__rust_stack_shim (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0x5BFBAEB: upcall_call_shim_on_rust_stack (rust_upcall.cpp:103)
==936==    by 0xABC0B7: rt::uv::net::__extensions__::connect_cb::_bca9d17c2f9fc276::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0x5C0FD95: uv__stream_io (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/stage1/lib/rustc/x86_64-unknown-linux-gnu/lib/librustrt.so)
==936==    by 0x5C162BC: uv__io_poll (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/stage1/lib/rustc/x86_64-unknown-linux-gnu/lib/librustrt.so)
==936==    by 0x5C0B40D: uv_run (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/stage1/lib/rustc/x86_64-unknown-linux-gnu/lib/librustrt.so)
==936==    by 0xAB33D4: rust_uv_run__c_stack_shim (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==  Address 0x1abd4100 is 9,998,528 bytes inside a block of size 10,000,048 alloc'd
==936==    at 0x4C2B4F0: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==936==    by 0x5BFF2EE: rust_exchange_alloc::realloc(void*, unsigned long) (rust_exchange_alloc.cpp:32)
==936==    by 0x5BF37FF: vec_reserve_shared (rust_util.h:75)
==936==    by 0x4248F9: vec::rustrt::vec_reserve_shared::_d93fcb2da81ef4d::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0x4246AA: vec::reserve_11902::_feb243f343efb87f::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0x749204: vec::with_capacity_20601::_d069f83199a42fd0::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xAC5255: rt::stack::__extensions__::meth_41483::new::_2f39639a42826edf::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA74579: rt::stack::__extensions__::meth_39135::take_segment::_2f39639a42826edf::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA73DE6: rt::sched::__extensions__::meth_39126::with_local::_03fe2be4efca6ed::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA84686: rt::test::spawntask_immediately::_866c76cd73dffbc7::_07pre (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA85A23: rt::io::net::tcp::test::multiple_connect_serial::anon::expr_fn_39598 (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)
==936==    by 0xA74E95: rt::sched::__extensions__::build_start_wrapper::anon::anon::expr_fn_39150 (in /opt/dev/rust3/build/x86_64-unknown-linux-gnu/test/coretest.stage1-x86_64-unknown-linux-gnu)

[brson]

Fixed.