Support returning mutable references by using returns

[audunhalland]

#49 added support for mutable references from AnswerFn.

Adding support for .returns should be possible, but some technically backwards-incompatible changes are needed.

Accessing the output value of a return responder is done using a method called get_output(&self). To add mutation support it needs another method like get_output_mut(&mut self) and the eval function also needs its own mutable version (eval_mut(&mut Unimock, ..)) to be able to call the new method.

edit: also consider supporting fragile returns, but not 100% sure. Fragile values are more likely to cause problems when used as returns compared to created in answer fns. For example, a cloned Unimock used from another thread won't work.