tethys v0.6.3.1

Few minor bug fixes:

• fix derivation for sealed structures (#23)
• Rename artifacts (#24)

tethys v0.6.3

New features:

1. Renaming syntax for JsonWriter builder. Code example ( #15 )
2. String tokenizer for protocol tests. Code example (#13)

Bug fixes : #5, #7, #14

v0.6.2

Fix misspelling in README.md