filter branches (incomplete)

Promosing work toward metadata driven filter branches. A few methods
to construct them are stubbed out; all the data types and pure code
seems good.

This commit was sponsored by Walter Somerville.

Types/MetaData.hs

@@ -16,6 +16,8 @@ module Types.MetaData (
 	deserialize,
 	MetaSerializable,
 	toMetaField,
+	mkMetaField,
+	tagMetaField,
 	fromMetaField,
 	toMetaValue,
 	mkMetaValue,
@@ -225,6 +227,9 @@ mkMetaField f = maybe (Left $ badField f) Right (toMetaField f)
 badField :: String -> String
 badField f = "Illegal metadata field name, \"" ++ f ++ "\""
 
+tagMetaField :: MetaField
+tagMetaField = MetaField "tag"
+
 {- Avoid putting too many fields in the map; extremely large maps make
  - the seriaization test slow due to the sheer amount of data.
  - It's unlikely that more than 100 fields of metadata will be used. -}