Constructors for constrained shapes that bypass validation

[david-perez]

Users have expressed interest in having a ConstrainedString::from_unchecked("invalid string") constructor that bypasses validation, either because they're absolutely sure that the data is valid or they have a reason to work with invalid data (for example, say a constraint is added to the model, and the service needs to return a value that was stored in the database prior to the model update; they want to grandfather-in these values).