`derefs_to` should not requires `ExpectedT` to be `Copy`

The implementation of `Matcher` on `DefersTo` requires the `ExpectedT` to be `Copy`:
https://github.com/google/googletest-rust/blob/290791e6424349fa136b34eb295c5efb5d5179bd/googletest/src/matchers/derefs_to_matcher.rs#L42-L59

This trait bound is not necessary as we actually need `&ExpectedT : Copy + Debug` (due to `Matchers`'s trait bound) and [a reference is always `Copy`](https://doc.rust-lang.org/std/primitive.reference.html#trait-implementations-1)