remove def SID field

DefSpec now only has the DefKey fields
Merge pull request #13 from sourcegraph/remove-SID

remove def SID field
rename DefAuthors Chars fields to Bytes and add sort impl
Defs.ListRefs
Remove units
remove all units!
update List comment
Revert "update List comment"

This reverts commit cbe677d3bb159211a99f5ca67644ed0534908f8f.