Add lifetimes to function types (new version of Rust requires this).

Signed-off-by: Atheriel <atheriel@gmail.com>
Start claiming rust-image as a dependency. No more home-cooked PGM.

The examples have been reworked to use the rust-image API and produce
PNG files --- this is much better than converting them afterwards.

It runs much, much faster as well.

Signed-off-by: Atheriel <atheriel@gmail.com>