Macro impl_writeable_tlv_based_enum

macro_rules! impl_writeable_tlv_based_enum {
    ($st: ident, $(($variant_id: expr, $variant_name: ident) =>
		{$(($type: expr, $field: ident, $fieldty: tt)),* $(,)*}
	),* $(,)*;
	$(($tuple_variant_id: expr, $tuple_variant_name: ident)),*  $(,)*) => { ... };
}

Implement Readable and Writeable for an enum, with struct variants stored as TLVs and tuple variants stored directly. The format is, for example

impl_writeable_tlv_based_enum!(EnumName,
  (0, StructVariantA) => {(0, required_variant_field, required), (1, optional_variant_field, option)},
  (1, StructVariantB) => {(0, variant_field_a, required), (1, variant_field_b, required), (2, variant_vec_field, optional_vec)};
  (2, TupleVariantA), (3, TupleVariantB),
);

The type is written as a single byte, followed by any variant data. Attempts to read an unknown type byte result in DecodeError::UnknownRequiredFeature.