The proper name of the library is `annotate-snippet`, not `annotate-rs`,
this commit should get rid of any confusing `AnnotateRs` names.
1. Renames `annotate_rs_emitter.rs` to
`annotate_snippet_emitter_writer.rs` so that the difference between the
`Emitter` trait and the implementers is more clear.
2. Renames `AnnotateRsEmitterWriter` to `AnnotateSnippetEmitterWriter`
3. Renames `HumanReadableErrorType::AnnotateRs` to `HumanReadableErrorType::AnnotateSnippet`
