Added missing values in std::io::standard_error()
add BigUint subtraction underflow error message
Rewrote documentation for parse_bytes and to_str_bytes in {int, uint}_macros.rs
auto merge of #13792 : jacob-hegna/rust/master, r=alexcrichton

Just modified the documentation for parse_bytes to make it more clear how the bytes were parsed (big endian) and to show an example of what it returned.  I also added documentation for the to_str_bytes which previously had no documentation (besides one stackoverflow post).
auto merge of #13799 : m-r-r/rust/patch-std-io-standard_error, r=alexcrichton

Hello,

With the latest version of Rust, calling to the function [`std::io::standard_error()`](http://static.rust-lang.org/doc/master/std/io/fn.standard_error.html) succeeds only if the value of the argument is `EndOfFile`, `IoUnavailable` or `InvalidInput`. If the function is called with another value as argument, it fails without message.

Here is a piece of code that reproduces the problem:

```rust
use std::io::{standard_error,EndOfFile,FileNotFound,PermissionDenied};

fn main() {
     println!("Error 1: {}", standard_error(EndOfFile)); // does not fail
     println!("Error 2: {}", standard_error(FileNotFound)); // fails
     println!("Error 3: {}", standard_error(PermissionDenied)); //fails
}
```
This was because the `IoErrorKind` passed as argument wasn't matched against all the possible values.

I added the missing branches in the `match` statement inside the function, and i removed the call to the `fail!()` macro. I rebuilt the crate with the latest `rustc` version and it seems to works.
auto merge of #13801 : ryantm/rust/master, r=alexcrichton

The previous error message using assert_eq! was quite cryptic. This should be more clear. I also added a test for the underflow case.