Immutable types

**Is your feature request related to a problem? Please describe.**
Would be nice if the types generated were immutable (even if they remain `CLIMutable`). I understand that minimizing copying might have been the goal, but there gotta be a relatively inexpensive way to implement `x.ReadFrom` that doesn't involve mutation (at least I think that's where the mutability comes in).

**Describe the solution you'd like**
I'll take any.

**Describe alternatives you've considered**
Live with the mutable types.

**Additional context**
n/a.