use smartstring's deserializer to support non borrowed strings

The map visitor for Dynamic was expecting a &str for the key, but the
serde_json deserializer internally uses a Cow string, which can be
Borrowed or Owned. In the case of Owned, the serde_json key deserializer
is calling visit_string on the Visitor, which for &str will result in
the error:
Error("invalid type: string \"a\", expected a borrowed string", line: 0,
column: 0)

smartstring actually has its own Visitor implementation that handles
both cases, so we can use it instead of an explicit conversion.

src/serde/deserialize.rs

@@ -143,8 +143,8 @@ impl<'d> Visitor<'d> for DynamicVisitor {
     fn visit_map<M: serde::de::MapAccess<'d>>(self, mut map: M) -> Result<Self::Value, M::Error> {
         let mut m = crate::Map::new();
 
-        while let Some((k, v)) = map.next_entry::<&str, _>()? {
-            m.insert(k.into(), v);
+        while let Some((k, v)) = map.next_entry()? {
+            m.insert(k, v);
         }
 
         Ok(m.into())