emacs mode indendation is wrong for string constants with backslashes

[nikomatsakis]

the following code indents oddly:

pub fn foo() {
    format!("abc \
def")
}

I expect:

pub fn foo() {
    format!("abc \
             def")
}

[nikomatsakis]

cc @pnkfelix

[huonw]

cc #8793

[shepmaster]

This now fails with the ever-so-useful Scan error: "Unbalanced parentheses"

[steveklabnik]

Moved to rust-lang/rust-mode#9