Add `-Z unstable-options` debugging flag, which can then be used to
extend the `rustc` command line interface with options that we do not
want to commit to making part of the long-term public interface for
`rustc`.
Add `--pretty everybody_loops` option.

This prints out a transformed version of the input source code where
every function body is replaced with `loop { }`.

All such bodies are (1.) trivial and (2.) guaranteed to pass the
type-checker in a valid compiler; therefore they make very nice input
to start with when narrowing down a bug exposed by a large test input
(such as librustc itself).
Add an unstable `--xpretty _` option to `rustc`.  Moved `flowgraph`
and `everybody_loops` options to `--xpretty`.