Add readlines, deletelines, and move some code
Move test to suit testee's new location
Fix bug in finditer
Give all deleting methods a return_deleted arg.

Also remove methods that have become redundant as a result.
Gereralize deleting up to any search match
Kill (right|left)_part methods, use subsegment!
Make the parse_slice method a separate class
Init with start, stop instead of start, size.
Make Slice min and max explicit
Use Slice in SegmentChain.delete, fix bugs
Light intro in README.md