Allow non-copyable Return types in with-style methods

[czechboy0]

Motivation

Fixes #77.

Users might create a value of a non-copyable type and return it from one of the watch methods. Today, that's not possible, because the Return type implicitly requires Copyable. But that's just because it's the default, we don't do any copying in the watch methods themselves, so we can relax it.

Modifications

Allow returning non-copyable values from watch methods.

Result

Easier usage of non-copyable types with Swift Configuration.

Test Plan

Adapted tests, all pass.