Improve some names.

Lots of vectors of messages called `message` or `msg`. This commit
pluralizes them.

Note that `emit_message_default` and `emit_messages_default` both
already existed, and both process a vector, so I renamed the former
`emit_messages_default_inner` because it's called by the latter.

Author: nnethercote

src/parse/session.rs

@@ -370,7 +370,7 @@ mod tests {
 
         fn build_diagnostic(level: DiagnosticLevel, span: Option<MultiSpan>) -> Diagnostic {
             let mut diag = Diagnostic::new(level, "");
-            diag.message.clear();
+            diag.messages.clear();
             if let Some(span) = span {
                 diag.span = span;
             }