Rework StringPart.

When there are two possibilities, both of which use a `String`, it's
nicer to use a struct than an enum. Especially when mapping the contents
into a tuple.

compiler/rustc_errors/src/snippet.rs

@@ -197,7 +197,7 @@ pub struct StyledString {
     pub style: Style,
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Hash, Encodable, Decodable)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
 pub enum Style {
     MainHeaderMsg,
     HeaderMsg,