Implement Rope::delete()

[hawkw]

This should be reasonably easy now that we have a working Rope::split() implementation.

Wikipedia says:

This operation can be done by two Split() and one Concat() operation. First, split the rope in three, divided by i-th and i+j-th character respectively, which extracts the string to delete in a separate node. Then concatenate the other two nodes.

[hawkw]

Implemented by @twisted-pear in #44