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).
Move std::rt::io to std::io