librustc: Change the syntax of subslice matching to use postfix ..

instead of prefix `..`.

This breaks code that looked like:

    match foo {
        [ first, ..middle, last ] => { ... }
    }

Change this code to:

    match foo {
        [ first, middle.., last ] => { ... }
    }

RFC #55.

Closes #16967.

[breaking-change]

src/doc/rust.md

@@ -3300,7 +3300,7 @@ it will bind the corresponding slice to the variable. Example:
 fn is_symmetric(list: &[uint]) -> bool {
     match list {
         [] | [_]                   => true,
-        [x, ..inside, y] if x == y => is_symmetric(inside),
+        [x, inside.., y] if x == y => is_symmetric(inside),
         _                          => false
     }
 }

src/doc/tutorial.md

@@ -1707,7 +1707,7 @@ let score = match numbers {
     [] => 0,
     [a] => a * 10,
     [a, b] => a * 6 + b * 4,
-    [a, b, c, ..rest] => a * 5 + b * 3 + c * 2 + rest.len() as int
+    [a, b, c, rest..] => a * 5 + b * 3 + c * 2 + rest.len() as int
 };
 ~~~~