Add rope implementation based on AVL trees.

.gitignore

include/rope.h

src/rope.c

test/unit/CMakeLists.txt

test/unit/rope.c

test/unit/rope.result

test/unit/rope.test

test/unit/rope_avl.c

test/unit/rope_avl.result

test/unit/rope_avl.test

test/unit/rope_basic.c

test/unit/rope_basic.result

test/unit/rope_basic.test

test/unit/rope_common.h

test/unit/rope_stress.c

test/unit/rope_stress.result

test/unit/rope_stress.test