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/libcore/task/spawn.rs

@@ -127,7 +127,7 @@ type TaskGroupInner = &'self mut Option<TaskGroupData>;
 
 // A taskgroup is 'dead' when nothing can cause it to fail; only members can.
 pure fn taskgroup_is_dead(tg: &TaskGroupData) -> bool {
-    (&tg.members).is_empty()
+    (&const tg.members).is_empty()
 }
 
 // A list-like structure by which taskgroups keep track of all ancestor groups