Extract GET operations into separate module

- Create src/get_operations.rs with all read operations
- Move get(), get_mut(), contains_key(), get_or_default(), get_item(), try_get(), get_many()
- Move arena access methods: get_leaf(), get_leaf_mut(), get_branch(), get_branch_mut()
- Move LeafNode and BranchNode get methods and find_child_index()
- Add comprehensive tests for all GET operations
- Remove duplicate methods from lib.rs
- All tests pass (34/34) - 8 new tests added for GET operations

Author: KentBeck