Reduce the size of scheduler thread stacks

[brson]

They are 100KB right now, arbitrarily chosen. See how small they can be.

[nikomatsakis]

Not critical for 0.6; removing milestone

[june0cho]

I tested as 16KB by changing SCHED_STACK_SIZE = 1024*16 in rt/rust_sched_launcher.cpp file.
(PTHREAD_STACK_MIN was 16KB)
It works well.

[thestinger]

Triage bump, no progress on this yet.

[pnkfelix]

visiting for triage, email from 2013 sep 23. Not sure if this is still relevant in the new runtime, since we do not yet have segmented stack support there?

[thestinger]

Support for segmented stacks has been dropped, so there's no point worrying about this anymore.