Implement slicing syntax.

`expr[]`, `expr[expr..]`, `expr[..expr]`,`expr[expr..expr]`

Uses the Slice and SliceMut traits.

Allows ... as well as .. in range patterns.

src/librustc_back/svh.rs

@@ -245,6 +245,7 @@ mod svh_visitor {
         SawExprAssign,
         SawExprAssignOp(ast::BinOp),
         SawExprIndex,
+        SawExprSlice,
         SawExprPath,
         SawExprAddrOf(ast::Mutability),
         SawExprRet,
@@ -279,6 +280,7 @@ mod svh_visitor {
             ExprField(_, id, _)      => SawExprField(content(id.node)),
             ExprTupField(_, id, _)   => SawExprTupField(id.node),
             ExprIndex(..)            => SawExprIndex,
+            ExprSlice(..)            => SawExprSlice,
             ExprPath(..)             => SawExprPath,
             ExprAddrOf(m, _)         => SawExprAddrOf(m),
             ExprBreak(id)            => SawExprBreak(id.map(content)),