Fix associated data serialization to always include length

[marsella]

The conversion functions between AssociatedData and Vec<u8> only encode the data itself, but it's more misuse-resistant to encode the length prepended to the data. The encoding of the length can be a fixed size (see #470 for an upper bound).

• Update the TryFrom and From methods to convert between the two types
• At calling spots, remove any redundant length prepending