Use the BoundedVec to limit the supported number of refs in
RefAnnouncement messages. The size (235) is the maximum supported by
message wire size.
To reduce the complexity of the changes, BoundedVec is used again
instead of using a new bounded Refs or BTreeMap. This requires
defining new Decode/Encode logic for Ref's element types, and
workarounds for arbitrary instances.
Signed-off-by: Slack Coder <slackcoder@server.ky>