Added scan tracing.
Fixed bad tracing debug! statements.
Changed String formatting for pattern ast nodes.

Specifically, types were output as their AST nodes, which were hideously
unreadable.  I've fallen back to just "$ty" for the moment.
Fixed invalid tracing generation.

I used "format!" instead of "debug!".  Whoopsie.
Anonymous captures now work.

Also fixed two other tests where the needed syntax has changed.

Closes issue #15.