As described in aarondl/sqlboiler#905, sqlboiler converts uint64 to int64 in relation-methods. So I've fixed Uint64.Scan() to convert received int64 to uint64 when it is smaller than 0.

Also, I've done some works that aren't related to the issue directly.

• Add a comment to the fix introduced in Fix for uint64 value bigger than 1<<63 #8 for clarification
• Updated go.sum
  • It's been updated when I ran go mod tidy
  • I think these lines should be committed to the repository, for cache verification
    • cf. https://golang.org/ref/mod#go