librustc: Implement lifetime elision.

[pcwalton]

This implements RFC 39. Omitted lifetimes in return values will now be
inferred to more useful defaults, and an error is reported if a lifetime
in a return type is omitted and one of the two lifetime elision rules
does not specify what it should be.

This primarily breaks two uncommon code patterns. The first is this:

unsafe fn get_foo_out_of_thin_air() -> &Foo {
    ...
}

This should be changed to:

unsafe fn get_foo_out_of_thin_air() -> &'static Foo {
    ...
}

The second pattern that needs to be changed is this:

enum MaybeBorrowed<'a> {
    Borrowed(&'a str),
    Owned(String),
}

fn foo() -> MaybeBorrowed {
    Owned(format!("hello world"))
}

Change code like this to:

enum MaybeBorrowed<'a> {
    Borrowed(&'a str),
    Owned(String),
}

fn foo() -> MaybeBorrowed<'static> {
    Owned(format!("hello world"))
}

Closes #15552.

[breaking-change]

r? @nick29581

[nrc]

this note and the one above seem inappropriate now. They should probably be removed (from the test, but also from the compile, if possible)

[pcwalton]

Could you explain in more detail? It doesn't seem immediately inappropriate to me.

[nrc]

Oh sorry, I missed that there was no lifetime on the arg. Ignore that comment

[nrc]

looks good r=me with the test changes discussed on irc.

[bluss]

Disallowing naked -> &Foo would close issue #2478