Remove ExprSlice by hacking the compiler

[breaking-change]

The `mut` in slices is now redundant. Mutability is 'inferred' from position. This means that if mutability is only obvious from the type, you will need to use explicit calls to the slicing methods.

src/librustc_back/svh.rs

@@ -246,7 +246,6 @@ mod svh_visitor {
         SawExprAssign,
         SawExprAssignOp(ast::BinOp),
         SawExprIndex,
-        SawExprSlice,
         SawExprRange,
         SawExprPath,
         SawExprAddrOf(ast::Mutability),
@@ -280,7 +279,6 @@ mod svh_visitor {
             ExprField(_, id)         => SawExprField(content(id.node)),
             ExprTupField(_, id)      => SawExprTupField(id.node),
             ExprIndex(..)            => SawExprIndex,
-            ExprSlice(..)            => SawExprSlice,
             ExprRange(..)            => SawExprRange,
             ExprPath(..)             => SawExprPath,
             ExprAddrOf(m, _)         => SawExprAddrOf(m),