NamespaceId, SubspaceId, PayloadDigest, IsAuthorisedWrite, Entry, AuthorisedEntry

[sgwilym]

• Added NamespaceId trait
• Added SubspaceId trait
• Added PayloadDigest trait
• Entry is just a struct, rather than a trait.
  • Just uses normal generics rather than const generics, as these are constrained to a few types.
• AuthorisedEntry is a struct too.
  • There is no PossiblyAuthorisedEntry.
  • AuthorisedEntry::new uses the IsAuthorisedWrite trait (implementing Fn) to determine whether it should return an error or not.