rustc: implement error messages from lifetime elision RFC

[aturon]

The new lifetime elision rules recently landed, but the error messages for the rules do not reflect the RFC. The function

fn test(s: &str, t: &str) -> &str {
    s
}

produces the error

elision.rs:1:30: 1:34 error: missing lifetime specifier [E0106]
elision.rs:1 fn test(s: &str, t: &str) -> &str {
                                          ^~~~

The error messages are critical for providing a gentle slope from the intuition of borrowing to the mechanism of lifetimes.

cc @pcwalton @nick29581

[alexcrichton]

Nominating.

[pnkfelix]

Assigning P-high, 1.0 milestone. (Its relatively high priority because the RFC was accepted in part because of the error message support it proclaimed.)

[nikomatsakis]

The test now produces the appropriate help:

<anon>:1:30: 1:34 error: missing lifetime specifier [E0106]
<anon>:1 fn test(s: &str, t: &str) -> &str {
                                      ^~~~
<anon>:1:34: 1:34 help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `s` or `t`