Potential code optimization paths

[forehalo]

• map code clean map code clean #21
• narrow Item::{left/right} type from Node to Somr<Item> refactor: simplify item left/right refs #24
• refactor map implementation refactor: re-implement map type #26
• Update consumes [u8] instead of Vec<u8> refactor: decoder accept &[u8] as input #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)