Soundness issue in the Mutex API

[TethysSvensson]

This function can be compiled and leaks a reference to the stack:

fn bad(cs: &bare_metal::CriticalSection) -> &u32 {
    let x = bare_metal::Mutex::new(42u32);
    x.borrow(cs)
}

The solution is to change the type signature of borrow to:

pub fn borrow<'cs>(&'cs self, _cs: &'cs CriticalSection) -> &'cs T