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/test.rs

@@ -358,7 +358,11 @@ pub fn run_tests_console(opts: &TestOpts,
 
 fn print_failures(st: @ConsoleTestState) {
     st.out.write_line(~"\nfailures:");
-    let mut failures = st.failures.map(|t| t.name.to_str());
+    let mut failures = ~[];
+    for uint::range(0, vec::uniq_len(&const st.failures)) |i| {
+        let name = copy st.failures[i].name;
+        failures.push(name.to_str());
+    }
     sort::tim_sort(failures);
     for vec::each(failures) |name| {
         st.out.write_line(fmt!("    %s", name.to_str()));