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/libsyntax/print/pprust.rs

@@ -279,9 +279,10 @@ pub fn is_bol(s: @ps) -> bool {
 }
 
 pub fn in_cbox(s: @ps) -> bool {
-    let len = s.boxes.len();
+    let boxes = &*s.boxes;
+    let len = boxes.len();
     if len == 0u { return false; }
-    return s.boxes[len - 1u] == pp::consistent;
+    return boxes[len - 1u] == pp::consistent;
 }
 
 pub fn hardbreak_if_not_bol(s: @ps) { if !is_bol(s) { hardbreak(s.s); } }