Add: Implementation for ll_union()
Outsourced set type into own header file
Add: Implementation for r_set_union()
Added warning in set.h for partially done union operation in case of error
Add: Interface definition for ht_union()
Add stub for method avl_union()
Add helper function merge_tree() for avl_union()
Implement avl_union
Add ll_union.c to source files to build
Added union.c to CMakeLists.txt file
Fixup: Missing header
Fixup: ht_union() calls in union.c
Added missing cmakelists entry
Added missing header in union.c
Add: test for ll_union()
Add: Test for ll_union() with overlapping values
Add: Simple avl_union() test
Add: test for avl_union() with overlapping values