Refactor LambdaForm constructors

The LambdaForm class has several constructors, some of which do checks on the arguments, and some of which bypass checks because they are not needed. There is also a hierarchy of delegating constructors that provides default values for some of the arguments.

But, overall it's hard to at a glance see which constructor calls which, and which constructor will perform which checks.

Let's refactor these into static factories with clear names to make this clearer.