Rework logical type representation

[Ten0]

Resolves #22

What it does:

• Fixes logical type representation for fixed with logical type ({"type":{"name":"duration","type":"fixed","size":12},"logicalType":"duration"} -> {"type":"fixed", "name":"duration","size":12,"logicalType":"duration"}) This is what I had originally written but interop tests with apache-avro bamboozled me into thinking this is how it was supposed to be. It wasn't, and they later fixed it to make it homogeneous with Java.
• Removes support for overly nested types in schema deserialization. This means specifically that the old representation above won't parse. (And similar objects written by versions of apache-avro before their fix won't parse.) This is not supported by the reference Java implementation, and it's unspecified and unclear how references would resolve for named overly-nested types.
• Reworks logical type representation: SchemaNode goes from:

  pub enum SchemaNode {
    /// An Avro type that's not annotated with a logical type
    RegularType(RegularType),
    /// An Avro type that is annotated with a logical type
    LogicalType {
        /// The key of the [`RegularType`] (in the [`SchemaMut`]) that is
        /// annotated with this logical type
        inner: SchemaKey,
        /// The LogicalType this node is annotated with
        logical_type: LogicalType,
    },
  }

  to

  pub struct SchemaNode {
    /// The underlying regular type of this node
    pub type_: RegularType,
    /// Logical type that the avro type is annotated with, if any
    pub logical_type: Option<LogicalType>,
  }

  which is simpler to use, and used to be the representation before I got bamboozled into thinking it was not the appropriate representation by interop tests with the broken apache-avro.