Validators implicitly require value

[mausch]

IValidationFunctions has many functions that implicitly add a "required" constraint from their type.

For example: Float is string Formlet -> float Formlet, but should be string Formlet -> float option Formlet since the user might not submit a value. There should also be a function 'a option Formlet -> 'a Formlet validating a required value.

In short, I need to decouple requiredness validation,