Added tests to Zipper and Rope and optimized Zipper.findBy

ZipperTest:
* mapping on zipper and stream
* nextC behaviour
* modify behiavoiour
* next, previous, nextC, previousC - focus, lefts, rights change
* nextOr, deleteRightOr, deleteLeftOr, findZor, previousOr etc tests
* findZ shouldn't change elements
* tests for insertion
* length
* withFocus, position
* start, end, atStart, atEnd
* stackoverflow tests for: move, findBy, findNext, findPrevious

RopeTest:
* Tests for constructors (Rope.fromString, Rope.apply, Rope.empty)
* Tests for iterator and reverseIterator
* Stream <-> Rope round trip test.

Zipper:
* Optimized Zipper findBy method.
Merge pull request #522 from bartosz-witkowski/zipper-and-rope-tests

Zipper and Rope tests