Allow using smartpointers with source_diagnostic

[TheNeikos]

Currently only a simple borrow is performed in some cases, and as_ref in other. AsRef is however not always implemented and &Box<dyn Diagnostic> does not implement Diagnostic. This patch makes use of Borrow that correctly handles both cases.

Problems before:

error[E0599]: the method `as_ref` exists for reference `&AnErr`, but its trait bounds were not satisfied
   --> tests/test_diagnostic_source_macro.rs:14:17
    |
5   | struct AnErr;
    | ------------- doesn't satisfy `AnErr: AsRef<_>`
...
14  | #[derive(Debug, miette::Diagnostic, thiserror::Error)]
    |                 ^^^^^^^^^^^^^^^^^^ method cannot be called on `&AnErr` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `AnErr: AsRef<_>`
            which is required by `&AnErr: AsRef<_>`
note: the following trait must be implemented
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `as_ref`, perhaps you need to implement it:
            candidate #1: `AsRef`
    = note: this error originates in the derive macro `miette::Diagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `Box<(dyn Diagnostic + 'static)>: Diagnostic` is not satisfied
  --> tests/test_diagnostic_source_macro.rs:29:17
   |
29 | #[derive(Debug, miette::Diagnostic, thiserror::Error)]
   |                 ^^^^^^^^^^^^^^^^^^ the trait `Diagnostic` is not implemented for `Box<(dyn Diagnostic + 'static)>`
   |
   = note: required for the cast to the object type `dyn Diagnostic`
   = note: this error originates in the derive macro `miette::Diagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `Arc<(dyn Diagnostic + 'static)>: Diagnostic` is not satisfied
  --> tests/test_diagnostic_source_macro.rs:33:17
   |
33 | #[derive(Debug, miette::Diagnostic, thiserror::Error)]
   |                 ^^^^^^^^^^^^^^^^^^ the trait `Diagnostic` is not implemented for `Arc<(dyn Diagnostic + 'static)>`
   |
   = note: required for the cast to the object type `dyn Diagnostic`
   = note: this error originates in the derive macro `miette::Diagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)

[zkat]

Sorry, I seem to have forgotten about this PR. Thanks again for the patch!