remove extra::rope

It's broken/unmaintained and needs to be rewritten to avoid managed
pointers and needless copies. A full rewrite is necessary and the API
will need to be redone so it's not worth keeping this around.

Closes #2236, #2744
auto merge of #7629 : thestinger/rust/rope, r=graydon

It's broken/unmaintained and needs to be rewritten to avoid managed
pointers and needless copies. A full rewrite is necessary and the API
will need to be redone so it's not worth keeping this around (#7628).

Closes #2236, #2744