condition! macro should support multiple declarations

[ghost]

It would be nice to be able to write

condition! {
    foo: int -> ~str;
    bar: char -> bool;
}

instead of

condition! { foo: int -> ~str; }
condition! { bar: char -> bool; }

for modules that define a number of conditions.

This should only require a small change to libsyntax/ext/expand.rs once macros are capable of expanding to multiple items, per #4021 and #3086.

[graydon]

I was initially going to do that, but then decided that the way it's organized now will permit attaching a doc-comment to each condition, where a multiple-condition-declaration would not (and we don't have any mechanism for transcribing comments from the arguments-to-macros). So I went with the current limited format.

Is this an acceptable explanation?

[ghost]

Makes sense to me.