Remove #[fixed_stack_segment] and #[rust_stack]

These two attributes are no longer useful now that Rust has decided to leave
segmented stacks behind. It is assumed that the rust task's stack is always
large enough to make an FFI call (due to the stack being very large).

There's always the case of stack overflow, however, to consider. This does not
change the behavior of stack overflow in Rust. This is still normally triggered
by the __morestack function and aborts the whole process.

C stack overflow will continue to corrupt the stack, however (as it did before
this commit as well). The future improvement of a guard page at the end of every
rust stack is still unimplemented and is intended to be the mechanism through
which we attempt to detect C stack overflow.

Closes #8822
Closes #10155
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.
auto merge of #10407 : alexcrichton/rust/no-fixed-stack-segment, r=brson

The commit messages have more details, but this removes all analysis and usage related to fixed_stack_segment and rust_stack attributes. It's now the assumption that we always have "enough stack" and we'll implement detection of stack overflow through other means.

The stack overflow detection is currently implemented for rust functions, but it is unimplemented for C functions (we still don't have guard pages).