Allow version-safe mutation of the data models in the v1.0.0 release

[popematt]

We need to take care with mutable models because we're currently using an un-versioned model, and direct modification of that model could produce an in memory model that is not valid in any ISL version (for example, a type that has a scale and exponent constraint, or adding the distinct modifier to an element constraint in a schema that is otherwise using ISL 1.0 features).

Some possibilities include:

• Convert the data model back into a type-safe builder for modification
• Add de-structuring functions to make it easy to take ownership of the data in a constraint, modify it, and then create a new constraint using that data
• Have a versioned data model (which we previously decided against, but could revisit)

[popematt]

An alternate approach that may work for some use cases would be to clone() the builder before calling build() so that you have a pre-constructed builder that you can continue modifying.