Add an iterator for a subpath's commands
Add an iterator for a subpath's commands

Generally this lets code that operates on subpaths *not* have to deal
with iterating the coords manually.

Also, it will allow us to move all the to_cairo to drawing_ctx.rs,
without that code having to have access to Path's internals and its coords.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg