Move diagnostics reporting to annotate_snippets

[W95Psp]

Since #735 Rustc doesn't allow us to report diagnostics with custom error codes any more.
This was needed for #277 anyway, and decoupling rustc sessions and error reporting will help in general with the design of the CLI.

[W95Psp]

Miette was not the best candidate, since it's output format is very different from the one used in Rustc.
Also, Rustc exposes its own reporting crate, annotate_snippets, so let's use that instead.