syntax: Display spans for open delimiters when a file ends prematurely

It's more helpful to list the span of each open delimiter seen so far
than to print out an error with the span of the last position in the file.

Closes #2354
syntax: Remove unnecessary @
auto merge of #9756 : catamorphism/rust/issue-2354, r=alexcrichton

r? anybody It's more helpful to list the span of each open delimiter seen so far
than to print out an error with the span of the last position in the file.

Closes #2354
Implement a concat!() format extension

This extension can be used to concatenate string literals at compile time. C has
this useful ability when placing string literals lexically next to one another,
but this needs to be handled at the syntax extension level to recursively expand
macros.

The major use case for this is something like:

    macro_rules! mylog( ($fmt:expr $($arg:tt)*) => {
        error2!(concat!(file!(), ":", line!(), " - ", $fmt) $($arg)*);
    })

Where the mylog macro will automatically prepend the filename/line number to the
beginning of every log message.