add StructureBuilder missing python methods

[rbran]

[rbran]

Could the non-mutable setters be problematic? What if the user hold a reference to the builder (or internals) before a setter call?

Maybe a code like this could cause problems:

let builder: StructureBuilder = todo!();
let members = builder.members();
builder.clear_members();
for member in members.iter() {
 ...
}

This could be solved by having all the setters functions require &mut self and return an Array tied to the self lifetime. Or simply clone all the members and return a Vec<StructureMember> instead.

[ElykDeer]

This isn't building locally for me... Complaining about CoreArrayWrapper not accepting a lifetime parameter. Not sure why it works in CI, though. I'll look into this more later.

[rbran]

This was written before the GAT PR, I'll fix this.

[rbran]

@KyleMiles Fixed.