Make from_reader and to_writer generic

[kirawi]

from_reader now takes a builder parameter that implements Write, and to_writer will accept any Iterator<Item = &str>. One consideration is that using from_reader with RopeWrite means needing to manually finish() it. The majority of the diff is from moving these two functions into a separate file. The changes are otherwise a ~15LoC change to the implementation and changing the function calls to reflect the changes.

[pascalkuthe]

Just out of curiosity: Why are you implementing this? Do we need anything else but a rope anywhere?

[kirawi]

#3966 needed to reimplement from_reader for a String.

[archseer]

from_reader is specialized for ropes, #3966 should just std::fs::read and then decode the contents

[archseer]

Or RopeBuilder could support something like Writer then finish would be called outside that from_reader method