Spark coin deserialization should enforce fixed sizes

[AaronFeickert]

Spark coins contain encrypted data whose size is fixed by the coin type:

• mint coin encrypted data should occupy 84 bytes: 16-byte encrypted diversifier, 32-byte encoded scalar, 32-byte padded memo
• spend coin encrypted data should occupy 90 bytes: 8-byte encoded value, 16-byte encrypted diversifier, 32-byte encoded scalar, 32-byte padded memo

These sizes should be enforced.