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/smallintmap.rs

@@ -50,18 +50,19 @@ impl<V> ReverseIter<(uint, &'self V)> for SmallIntMap<V> {
 
 impl<V> Container for SmallIntMap<V> {
     /// Return the number of elements in the map
-    pure fn len(&self) -> uint {
+    pure fn len(&const self) -> uint {
         let mut sz = 0;
-        for self.v.each |item| {
-            if item.is_some() {
-                sz += 1;
+        for uint::range(0, vec::uniq_len(&const self.v)) |i| {
+            match self.v[i] {
+                Some(_) => sz += 1,
+                None => {}
             }
         }
         sz
     }
 
     /// Return true if the map contains no elements
-    pure fn is_empty(&self) -> bool { self.len() == 0 }
+    pure fn is_empty(&const self) -> bool { self.len() == 0 }
 }
 
 impl<V> Mutable for SmallIntMap<V> {