tutorial.md : correct formula in fn area()

change formula (other solution could be using abs()).
#7752: use fcnptr for glob errfunc.
Rename from_utf8 to from_bytes again
errfunc ptr is nullable, so use Option as part of interface to glob (#7752).
Refactor the logging system for fewer allocations

This lifts various restrictions on the runtime, for example the character limit
when logging a message. Right now the old debug!-style macros still involve
allocating (because they use fmt! syntax), but the new debug2! macros don't
involve allocating at all (unless the formatter for a type requires allocation.
Fix visibility of digest implementations
Some struct visibility fixes
std::rt: Implement task yielding. Fix a starvation problem
extra::ringbuf: Implement method `.swap(uint, uint)` just like vector

RingBuf::swap(&mut self, i, j) swaps the element at indices `i` and `j`
if both elements are in bounds, otherwise it fails.
Regression test for ICE.  Fix #5153.
auto merge of #9497 : pnkfelix/rust/fsk-7752-use-fcnptr-for-glob-errfunc, r=cmr

Fix #7752.

~~(The glob API is a little funky; I tried to make a small test for it, which I'll add to the end of this description, and its not clear whether globfree is supposed to free solely the structure allocated by glob itself, or if it is going to try to free more than that.)~~ (The previous note was a user-error: I was misusing the CString API.)

Anyway, this seems to work in terms of calling errfunc where expected.)

```rust
#[allow(unused_imports)];
use std::libc::types::os::arch::c95::{c_char, c_int, size_t};
use std::libc::funcs::posix01::glob;
use std::libc::types::os::common::posix01::glob_t;
use std::libc::consts::os::posix01::{GLOB_APPEND, GLOB_DOOFFS, GLOB_ERR,
                                     GLOB_MARK, GLOB_NOCHECK, GLOB_NOSORT,
                                     GLOB_NOESCAPE, GLOB_NOSPACE,
                                     GLOB_ABORTED, GLOB_NOMATCH};
use std::ptr;
use std::c_str;

#[fixed_stack_segment]
fn main() {
    let mut g = glob_t {
        gl_pathc:  0, // size_t,
        __unused1: 0, // c_int,
        gl_offs:   2, // size_t,
        __unused2: 0, // c_int,
        gl_pathv:  ptr::null(), // **c_char,

        __unused3: ptr::null(), // *c_void,

        __unused4: ptr::null(), // *c_void,
        __unused5: ptr::null(), // *c_void,
        __unused6: ptr::null(), // *c_void,
        __unused7: ptr::null(), // *c_void,
        __unused8: ptr::null(), // *c_void,
    };

    extern "C" fn errfunc(_epath: *c_char, _errno: int) -> int {
        println!("errfunc called");
        return 0;
    }

    struct Reduced { pathc: size_t, offs: size_t, pathv: **c_char, }
    impl Reduced {
        fn from(g: &glob_t) -> Reduced {
            Reduced {pathc: g.gl_pathc, offs: g.gl_offs, pathv: g.gl_pathv}
        }
    }

    do ("*.rs/*").with_c_str |pat| {
        println!("calling glob");
        unsafe { glob::glob(pat, GLOB_DOOFFS, errfunc, &mut g); }
        println!("After glob call");

        println!("g: {:?}", Reduced::from(&g));
        for i in range(0, g.gl_pathc as int) {
            unsafe {
                let p : **c_char = ptr::offset(g.gl_pathv, g.gl_offs as int + i);
                let x = c_str::CString::new(*p, false);
                match x.as_str() {
                    Some(s) => {
                        println!("gl_pathc[{:d}]: {:?}", i, s);
                    }
                    None => {
                        println!("gl_pathc[{:d}]: unvalid", i);
                    }
                }
            }
        }
    }

    println!("calling globfree on g: {:?}", g);
    unsafe { glob::globfree(&mut g); }
    println!("after globfree call");

}

```
auto merge of #9500 : fhahn/rust/rename-str_from_bytes-fix, r=alexcrichton

As @Dretch pointed out [here](https://github.com/fhahn/rust/commit/de39874801761197bf10bf8d04bde1aa2bd82e15#L2L526) , from_bytes was accidentally renamed to from_utf8.
Enclose code in documentation in backticks
base64: backticks for code in documentation
bigint: backticks for code in documentation
bitv: backticks for code in documentation
syntax: Fix wrong span on ObsoleteEmptyImpl
syntax: Fix wrong span on ObsoleteMode
syntax: Fix wrong span on trait fn visibility

Fixes #9348.
auto merge of #9503 : dcrewi/rust/fix-digest-visibility, r=alexcrichton

I really have no idea why the tests didn't fail. Maybe it's another cross-crate issue?