Make all lang_items optional

Whenever a lang_item is required, some relevant message is displayed, often with
a span of what triggered the usage of the lang item.

Now "hello word" is as small as:

```rust
#[no_std];

extern {
    fn puts(s: *u8);
}

extern "rust-intrinsic" {
    fn transmute<T, U>(t: T) -> U;
}

#[start]
fn main(_: int, _: **u8, _: *u8) -> int {
    unsafe {
        let (ptr, _): (*u8, uint) = transmute("Hello!");
        puts(ptr);
    }
    return 0;
}
```