Introduces:
- `SatInstance::var_manager_ref()`
- `SatInstance::var_manager_mut()`
- `SatInstance::n_vars()`
- `OptInstance::constraints_ref()`
- `OptInstance::constraints_mut()`
- `OptInstance::objective_ref()`
- `OptInstance::objective_mut()`
- `MultiOptInstance::constraints_ref()`
- `MultiOptInstance::constraints_mut()`
- `MultiOptInstance::objective_ref()`
- `MultiOptInstance::objective_mut()`

Deprecates:
- `SatInstance::var_manager()`
- `OptInstance::get_constraints()`
- `OptInstance::get_objective()`
- `MultiOptInstance::get_constraints()`
- `MultiOptInstance::get_objective()`

Make `to_opb` and similar opb writing functions take by reference
instead of by value. Some still need to take mutable references, provide
workarounds in documentation if additional guarantees are known.

Make `to_dimacs` and similar dimacs writing functions take by reference
instead of by value. For the high-level instance types mutable
references are still needed; provide `Cnf::to_dimacs` and workarounds in
documentation if additional guarantees are known.

Since external libraries cannot easily take ownership anyway, for most
solvers this is more reasonable.
Add a default implementation for `add_clause` that can be overwritten
for solvers that can truly take ownership.

- rename file writing methods `write_` rather than `to_`
- make all file writing methods take references and return errors if not
  in right format
- rename all heavy converters `into_`
- rename all inplace converters `convert_to_`

in tests and two methods of `Objective` type