initial `lib.rs` modularization

[alejandro-vaz]

split core and easy modules from lib.rs and start draining it

this includes:

• bytes.rs
• comparisons.rs: eq and ord, total and partial
• mallocsizeof.rs
• complete rawsmallvec.rs
• references.rs: asref, deref, borrow
• serde.rs
• taggedlen.rs