librustc_errors: Rename AnnotateRs -> AnnotateSnippet

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`
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 6a35906..003fdd5 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -2003,7 +2003,7 @@
             None |
             Some("human") => ErrorOutputType::HumanReadable(HumanReadableErrorType::Default(color)),
             Some("human-annotate-rs") => {
-                ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateRs(color))
+                ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateSnippet(color))
             },
             Some("json") => ErrorOutputType::Json { pretty: false, json_rendered },
             Some("pretty-json") => ErrorOutputType::Json { pretty: true, json_rendered },
@@ -2041,7 +2041,7 @@
                 "--error-format=pretty-json is unstable",
             );
         }
-        if let ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateRs(_)) =
+        if let ErrorOutputType::HumanReadable(HumanReadableErrorType::AnnotateSnippet(_)) =
             error_format {
             early_error(
                 ErrorOutputType::Json { pretty: false, json_rendered },