Extract EncodableRecord from MutablePersistableRecord

[groue]

This PR introduces a new record protocol, EncodableRecord, and redefines PersistableRecord on top of it:

// Defines how a record encodes itself into the database
protocol EncodableRecord {
    /// Defines the values persisted in the database
    func encode(to container: inout PersistenceContainer)
}

// Adds persistence methods
protocol MutablePersistableRecord: TableRecord, EncodableRecord {
    /// Optional method that lets your adopting type store its rowID upon
    /// successful insertion. Don't call it directly: it is called for you.
    mutating func didInsert(with rowID: Int64, for column: String?)
}

// Adds immutability
protocol PersistableRecord: MutablePersistableRecord {
    /// Non-mutating version of the optional didInsert(with:for:)
    func didInsert(with rowID: Int64, for column: String?)
}

An EncodableRecord type defines its database encoding, but remains deprived from the persistence methods granted by PersistableRecord. This makes it possible to use all Association APIs on read-only records.

Before this PR, one had to conform to PersistableRecord in order to use an association request(for:) method. #426 has identified this as an annoyance.

Despite the change in the record protocol hierarchy, this is a purely additive PR, which should not break any existing code. No test for public APIs had to be changed.