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/libcore/ptr.rs

@@ -367,7 +367,7 @@ mod externfnpointers {
             self_ == other_
         }
     }
-    macro_rules! fnptreq(
+    macro_rules! fnptreq {
         ($($p:ident),*) => {
             impl<_R,$($p),*> PartialEq for extern "C" fn($($p),*) -> _R {
                 #[inline]
@@ -379,12 +379,12 @@ mod externfnpointers {
                 }
             }
         }
-    )
-    fnptreq!(A)
-    fnptreq!(A,B)
-    fnptreq!(A,B,C)
-    fnptreq!(A,B,C,D)
-    fnptreq!(A,B,C,D,E)
+    }
+    fnptreq! { A }
+    fnptreq! { A,B }
+    fnptreq! { A,B,C }
+    fnptreq! { A,B,C,D }
+    fnptreq! { A,B,C,D,E }
 }
 
 // Comparison for pointers