Allow raw identifiers for SqlIdentifier (column-name)

This allows using the `r#identifier` syntax for SqlIdentifier
(column-name), which is used in derive macros.
Previously the derive macros would panic when encountering such an
identifier:
    `"r#identifier"` is not a valid identifier

diesel_derives/src/attrs.rs

@@ -75,7 +75,11 @@ impl SqlIdentifier {
 
 impl ToTokens for SqlIdentifier {
     fn to_tokens(&self, tokens: &mut TokenStream) {
-        Ident::new(&self.field_name, self.span).to_tokens(tokens)
+        if self.field_name.starts_with("r#") {
+            Ident::new_raw(&self.field_name[2..], self.span).to_tokens(tokens)
+        } else {
+            Ident::new(&self.field_name, self.span).to_tokens(tokens)
+        }
     }
 }