A few minor write_str optimizations and inlining

Apparently `write!` generates more code than `write_str` when used with a simple string, so optimizing it.

test_suite/tests/test_value.rs

@@ -63,7 +63,7 @@ fn test_map_access_to_enum() {
                 type Value = Potential;
 
                 fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
-                    write!(formatter, "a map")
+                    formatter.write_str("a map")
                 }
 
                 fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>