std::rt: 2MB stacks again
std::rt: Pull RUST_MIN_STACK from the environment
std: Allow spawners to specify stack size
rustc: Use 4MB stacks. Needed for unoptimized builds apparently.
rustpkg: Disable test_uninstall

Seems to not work
Turn on the new runtime
auto merge of #8304 : brson/rust/newrt, r=brson

Depends on #8303 and #8299.