Improve validation creation

[gaku-sei]

The current way to create a validation is, for instance:

let required =
  ( [ "required" ],
    fun { label; value } ->
      if String.isEmpty value then `error (`error ("required", label))
      else `ok value )

Which comes with 2 downsides:

• Verbose and bizarre: why do we need the [] around the "required" name?
• Unsafe: what if one just creates a validation with no name, or many names? (These could be an actual use case one day, but not for now)

There are several solutions to this problem:

• Use the NonEmptyList type under the hood
• Then create a function/operator to make validation creation simpler:

let required =
  "required" // fun { label; value } ->
    if String.isEmpty value then `error (`error ("required", label))
    else `ok value )

The above solution solves all the issues so far but it doesn't compile (the polymorphic type values is said "weak").