Add setters to object codegen

[sfackler]

We currently generate immutable structs for Conjure objects, and require them to be converted back into a builder to be modified. This matches the behavior of conjure-java, but feels unnecessary in Rust since it has more robust ownership and mutability semantics.

We should instead just add setters to the generated structs and leave the builders for initial construction.

[sfackler]

Alternatively, we could just make the fields public. That would also allow people to move out of them.