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,

[mausch]

I already did something about this for regex and email in 1e66d93

[mausch]

Related: remove optional parameter "required" in FormElements