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/priority_queue.rs

@@ -37,10 +37,10 @@ impl<T:Ord> BaseIter<T> for PriorityQueue<T> {
 
 impl<T:Ord> Container for PriorityQueue<T> {
     /// Returns the length of the queue
-    pure fn len(&self) -> uint { self.data.len() }
+    pure fn len(&const self) -> uint { vec::uniq_len(&const self.data) }
 
     /// Returns true if a queue contains no elements
-    pure fn is_empty(&self) -> bool { self.data.is_empty() }
+    pure fn is_empty(&const self) -> bool { self.len() == 0 }
 }
 
 impl<T:Ord> Mutable for PriorityQueue<T> {