Resolve bare_trait_objects warning

    warning: trait objects without an explicit `dyn` are deprecated
      --> src/ignore.rs:48:12
       |
    48 |         Ok(Visitor::ignore())
       |            ^^^^^^^ help: use `dyn`: `<dyn Visitor>`
       |
       = note: `#[warn(bare_trait_objects)]` on by default
       = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2021 edition!
       = note: for more information, see issue #80165 <rust-lang/rust#80165>

    warning: trait objects without an explicit `dyn` are deprecated
      --> src/ignore.rs:58:12
       |
    58 |         Ok(Visitor::ignore())
       |            ^^^^^^^ help: use `dyn`: `<dyn Visitor>`
       |
       = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2021 edition!
       = note: for more information, see issue #80165 <rust-lang/rust#80165>

src/ignore.rs

@@ -45,7 +45,7 @@ impl Visitor for Ignore {
 
 impl Seq for Ignore {
     fn element(&mut self) -> Result<&mut dyn Visitor> {
-        Ok(Visitor::ignore())
+        Ok(<dyn Visitor>::ignore())
     }
 
     fn finish(&mut self) -> Result<()> {
@@ -55,7 +55,7 @@ impl Seq for Ignore {
 
 impl Map for Ignore {
     fn key(&mut self, _k: &str) -> Result<&mut dyn Visitor> {
-        Ok(Visitor::ignore())
+        Ok(<dyn Visitor>::ignore())
     }
 
     fn finish(&mut self) -> Result<()> {