GitLab

codegen: assembler for struct with map representation now validates all non-optional fields are present.

This continues what https://github.com/ipld/go-ipld-prime/pull/111/ did
and adds the same logic to the map representation.  The actual state
tracking works the same way (and was mostly already there).

Rearranged the tests slightly.

Made error messages include both field name and serial key when they
differ due to a rename directive.  (It's possible this error would get
nicer if it used a list of StructField instead of just strings, but it
would also get more complicated.  Maybe revisit later.)