Potential code optimization paths

[forehalo]

• [x] map code clean #21
• [x] narrow Item::{left/right} type from Node to Somr<Item> #24
• [x] refactor map implementation #26
• [x] Update consumes [u8] instead of Vec<u8> #35
• [ ] make Item::Content inner easier to share, especially for Value::Read (perf)
• [ ] standardize Value::read(Content) for type content reading (perf/correctness)
• [ ] try separate map/list into branches (mem reduce)