Prepare omigrate for new driver

[maiste]

Hi!
This PR modifies the code interface to enable omigrate' to work with other DBSM`. The three main enhancements are:

• It makes port, user and password optional. For the postgres version, the default value is kept as there was in the command line to maintain compatibility. This is not breaking any change.
• It moves the Connection in Driver as they are related.
• Move the parse_uri function inside the Driver.S signature, as the parsing might not fail on the same parsing error.

(This is a prequel for #3)

[tmattio]

Thanks a lot @maiste! 🎉

Everything looks good to me, apart from the parse_uri part: I don't understand why it's needed to move it to the driver? Can you expand a little bit on this?

[maiste]

Thanks for your help with it 😉

About the parse_uri, the issue is mainly this part of the function:

match Uri.path uri with
      | "/" -> Error (Omigrate_error.bad_uri s)
      | path ->
          if Filename.dirname path <> "/" then Error (Omigrate_error.bad_uri s)
          else Ok (Filename.basename path)

We want to check this for the PostgreSQL driver but not for the sqlite3 driver, for which the check is different. I think the check would be different for another database system (NoSQL one, for example).

[tmattio]

Thanks!