refactor: improve Decode, Encode API

Summary

Closes #168 (closed)

• refactor: use byte slice instead of impl Read in Decode

• refactor: move EncodeStyle into Context

Depends on !416 (merged), which should be merged first

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files