Add support for including ignorable types in enum de/ser macros

An enum implements de/serialization via
`impl_writeable_tlv_based_enum_upgradable` currently cannot contain
an object that only implements `MaybeReadable`.

This solves that by implementing the required blocks for
`ignorable`, opting to return `Ok(None)` in the top-level read in
cases where the inner field read returns `Ok(None)`.

Author: TheBlueMatt

lightning/src/util/ser_macros.rs

@@ -39,6 +39,9 @@ macro_rules! _encode_tlv {
 			field.write($stream)?;
 		}
 	};
+	($stream: expr, $type: expr, $field: expr, ignorable) => {
+		$crate::_encode_tlv!($stream, $type, $field, required);
+	};
 	($stream: expr, $type: expr, $field: expr, (option, encoding: ($fieldty: ty, $encoding: ident))) => {
 		$crate::_encode_tlv!($stream, $type, $field.map(|f| $encoding(f)), option);
 	};
@@ -155,6 +158,9 @@ macro_rules! _get_varint_length_prefixed_tlv_length {
 			$len.0 += field_len;
 		}
 	};
+	($len: expr, $type: expr, $field: expr, ignorable) => {
+		$crate::_get_varint_length_prefixed_tlv_length!($len, $type, $field, required);
+	};
 }
 
 /// See the documentation of [`write_tlv_fields`].
@@ -581,6 +587,9 @@ macro_rules! _init_tlv_based_struct_field {
 	($field: ident, option) => {
 		$field
 	};
+	($field: ident, ignorable) => {
+		if $field.is_none() { return Ok(None); } else { $field.unwrap() }
+	};
 	($field: ident, required) => {
 		$field.0.unwrap()
 	};
@@ -610,6 +619,9 @@ macro_rules! _init_tlv_field_var {
 	($field: ident, option) => {
 		let mut $field = None;
 	};
+	($field: ident, ignorable) => {
+		let mut $field = None;
+	};
 }
 
 /// Equivalent to running [`_init_tlv_field_var`] then [`read_tlv_fields`].