MultiGlue

[KyGost]

MultiGlue feels like the feature GlueSQL was born to have!
What would make Glue really stand out would be a feature which allows one to interact across databases, specifically multiple types of databases.
I proposed it here: https://discordapp.com/channels/780298017940176946/780298017940176949/830597175841914920

The gist is, MultiGlue would enable tying together Glue instances of any type, be it Sled, Memory, CSV, ODBC or other, one could communicate between the databases.

An example usecase is as follows:

let sled = Glue::sled("sled/location");
let csv = Glue::csv("csv/location.csv");
let multiglue = Glue::multi(vec![("CSV", csv), ("Sled", sled)]);
multiglue.execute(parse_single(r#"
  INSERT INTO
    Sled.maintable (
      colA,
      colB,
      colC
    )
  SELECT
    A,
    B,
    C
  FROM
    CSV.data
"#))

The proposed change is, simply, to listen to three levels of reference:
[database].[table].[column] (as of current we listen to: [table].[column]) and choose which storage to run an operation on that basis.