Add optional metadata to trail().

[Brujo Benavides]

• create opaque type in trails called trail() and export it
• Modify trails:trail/1,2,3,4 and create trails:trail/5 by adding a Metadata::map() parameter, and they should return trails:trail() now.
• Make the internal representation for the opaque type trails:trail() include the metadata field
• Change the compiling functions so that they can now also receive a trail() and produce the expected output