feat: Generalize notes

[KaffinPX]

• Generalize notes into an extendable format with public support. (With format of [announcement visibility discriminant, note content type discriminant, content])
• Remove UTXO index. (Will be moved off-node, node will be maintaining only consensus-critical components)
• Standardize use of encode(), decode() on note content by BFEs). (Replacing old ciphertext using bincode
• Address decrypt fns now returns NoteContent directly.
• Some other "small" fixes...

(With a consequence; a typical output announcement size for generation address increased to 352 BFEs from 347 BFEs and symmetric address increased to 32 BFEs from 27 BFEs.)