Allow Codec#product to behave correctly with parameter count

[s5bug]

The way this is implemented is via a parameters: Int on Encoder.

Notes:

• The State[Int, String] solution from Skunk was attempted, but there was no way to prevent the bad behavior of (someNonUnit, Codec.unit).tupled from generating (?, ?, ?,) rather than (?, ?, ?). A possible solution to this issue is State[Int, List[Either[String, Int]]], where the transitions Left→Left, Left→Right, Right→Left are directly concatenated, but transitions Right→Right are joined with , .
• For some reason the string interpolator doesn't like ${ a *: b *: nil }, and I don't know why.
• It's not exactly clear how to handle Encoder#either in this case.
• I refactored the base types into a common superclass so I wouldn't have to write def parameters = 1 6 times.

[s5bug]

The latest commit addresses point 1, with the Fragment.Part ADT.

[s5bug]

JS support blocked by WiseLibs/better-sqlite3#1032