Some of the vec utilities now only work on immutable vecs, since they
would have to be rewritten to do a lot more copying to be alias-safe.
Some forced copying was added to map.rs, showing a weakness in the
alias checker (or maybe the alias system): when fn args are passed
into a function, calling them must assume all aliases that are not
immutably rooted (directly connected to a local or temporary without
any mutable edges) become invalid. This will be a drag on functional
programming in Rust.
Work around alias issues in the stdlib
