librustc: Make the compiler ignore purity.

For bootstrapping purposes, this commit does not remove all uses of
the keyword "pure" -- doing so would cause the compiler to no longer
bootstrap due to some syntax extensions ("deriving" in particular).
Instead, it makes the compiler ignore "pure". Post-snapshot, we can
remove "pure" from the language.

There are quite a few (~100) borrow check errors that were essentially
all the result of mutable fields or partial borrows of `@mut`. Per
discussions with Niko I think we want to allow partial borrows of
`@mut` but detect obvious footguns. We should also improve the error
message when `@mut` is erroneously reborrowed.

src/libstd/json.rs

@@ -757,12 +757,16 @@ pub fn Decoder(json: Json) -> Decoder {
 
 priv impl Decoder/&self {
     fn peek(&self) -> &'self Json {
-        if self.stack.len() == 0 { self.stack.push(&self.json); }
-        self.stack[self.stack.len() - 1]
+        if vec::uniq_len(&const self.stack) == 0 {
+            self.stack.push(&self.json);
+        }
+        self.stack[vec::uniq_len(&const self.stack) - 1]
     }
 
     fn pop(&self) -> &'self Json {
-        if self.stack.len() == 0 { self.stack.push(&self.json); }
+        if vec::uniq_len(&const self.stack) == 0 {
+            self.stack.push(&self.json);
+        }
         self.stack.pop()
     }
 }