librustc: Always parse macro!()/macro![] as expressions if not

followed by a semicolon.

This allows code like `vec![1i, 2, 3].len();` to work.

This breaks code that uses macros as statements without putting
semicolons after them, such as:

    fn main() {
        ...
        assert!(a == b)
        assert!(c == d)
        println(...);
    }

It also breaks code that uses macros as items without semicolons:

    local_data_key!(foo)

    fn main() {
        println("hello world")
    }

Add semicolons to fix this code. Those two examples can be fixed as
follows:

    fn main() {
        ...
        assert!(a == b);
        assert!(c == d);
        println(...);
    }

    local_data_key!(foo);

    fn main() {
        println("hello world")
    }

RFC #378.

Closes #18635.

[breaking-change]

src/libtest/stats.rs

@@ -459,14 +459,14 @@ mod tests {
     use std::io;
     use std::f64;
 
-    macro_rules! assert_approx_eq(
+    macro_rules! assert_approx_eq {
         ($a:expr, $b:expr) => ({
             use std::num::Float;
             let (a, b) = (&$a, &$b);
             assert!((*a - *b).abs() < 1.0e-6,
                     "{} is not approximately equal to {}", *a, *b);
         })
-    )
+    }
 
     fn check(samples: &[f64], summ: &Summary<f64>) {