Resolve some needless_lifetimes clippy lints

warning: the following explicit lifetimes could be elided: 'a
       --> serde/src/de/mod.rs:489:6
        |
    489 | impl<'a> Expected for &'a str {
        |      ^^                ^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
        |
    489 - impl<'a> Expected for &'a str {
    489 + impl Expected for &str {
        |

    warning: the following explicit lifetimes could be elided: 'a
       --> serde/src/de/mod.rs:495:6
        |
    495 | impl<'a> Display for Expected + 'a {
        |      ^^                         ^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
        |
    495 - impl<'a> Display for Expected + 'a {
    495 + impl Display for Expected + '_ {
        |

    warning: the following explicit lifetimes could be elided: 'a
        --> serde/src/de/mod.rs:1744:11
         |
    1744 | impl<'de, 'a, A> SeqAccess<'de> for &'a mut A
         |           ^^                         ^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
         |
    1744 - impl<'de, 'a, A> SeqAccess<'de> for &'a mut A
    1744 + impl<'de, A> SeqAccess<'de> for &mut A
         |

    warning: the following explicit lifetimes could be elided: 'a
        --> serde/src/de/mod.rs:1897:11
         |
    1897 | impl<'de, 'a, A> MapAccess<'de> for &'a mut A
         |           ^^                         ^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
         |
    1897 - impl<'de, 'a, A> MapAccess<'de> for &'a mut A
    1897 + impl<'de, A> MapAccess<'de> for &mut A
         |

    warning: the following explicit lifetimes could be elided: 'a, 'b
      --> serde/src/ser/fmt.rs:38:6
       |
    38 | impl<'a, 'b> Serializer for &'a mut fmt::Formatter<'b> {
       |      ^^  ^^                  ^^                    ^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
       |
    38 - impl<'a, 'b> Serializer for &'a mut fmt::Formatter<'b> {
    38 + impl Serializer for &mut fmt::Formatter<'_> {
       |

    warning: the following explicit lifetimes could be elided: 'a
      --> serde_derive/src/internals/symbol.rs:49:6
       |
    49 | impl<'a> PartialEq<Symbol> for &'a Ident {
       |      ^^                         ^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
       |
    49 - impl<'a> PartialEq<Symbol> for &'a Ident {
    49 + impl PartialEq<Symbol> for &Ident {
       |

    warning: the following explicit lifetimes could be elided: 'a
      --> serde_derive/src/internals/symbol.rs:61:6
       |
    61 | impl<'a> PartialEq<Symbol> for &'a Path {
       |      ^^                         ^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
    help: elide the lifetimes
       |
    61 - impl<'a> PartialEq<Symbol> for &'a Path {
    61 + impl PartialEq<Symbol> for &Path {
       |

serde_derive/src/internals/symbol.rs

@@ -46,7 +46,7 @@ impl PartialEq<Symbol> for Ident {
     }
 }
 
-impl<'a> PartialEq<Symbol> for &'a Ident {
+impl PartialEq<Symbol> for &Ident {
     fn eq(&self, word: &Symbol) -> bool {
         *self == word.0
     }
@@ -58,7 +58,7 @@ impl PartialEq<Symbol> for Path {
     }
 }
 
-impl<'a> PartialEq<Symbol> for &'a Path {
+impl PartialEq<Symbol> for &Path {
     fn eq(&self, word: &Symbol) -> bool {
         self.is_ident(word.0)
     }