Changes to make pcre bindings work with rust-master.

I don't know if this is backwards compatible at all.  It will also only work on a *very* recent version of rust-master, since that changes the structure of `rt::io::*` (hopefully it doesn't change again too soon)!  The tests all pass still.

Note that the `pkg.rs` script can no longer call `rust run`, so it instead calls `rustc` and then `./versioncheck`.

Note: I had to compile the demo manually (i.e. no `rustpkg`) because `rustpkg` kept erroring out when trying to save the output.  Apart from that, it appears to work!