Decrease the default stack size back to 2MB

I increased this to 4MB when I implemented abort-on-stack-overflow for Rust
functions. Now that the fixed_stack_segment attribute is removed, no rust
function will ever reasonably request 2MB of stack (due to calling an FFI
function).

The default size of 2MB should be plenty for everyday use-cases, and tasks can
still request more stack via the spawning API.