Allow adding generics to impl and trait name

[branchseer]

Prerequisites of bincode-org/bincode#710.

This PR allows

• adding generics after the trait name: impl<'a> Foo<&'a str> for StructOrEnum<'a>
• adding generics after the impl keyword: impl<T1, T2, Bar> Foo for StructOrEnum<T1, T2>

[branchseer]

Fix error raised by rust-clippy 0.1.81:

error: doc list item without indentation
   --> src/parse/body.rs:367:9
    |
367 |     /// In either case this value will be `Some(Literal::i32(5))`
    |         ^
    |

[VictorKoenders]

One minor formatting issue:

Diff in /home/runner/work/virtue/virtue/src/parse/body.rs:364:
     /// - `Baz = 5`
     /// - `Baz(i32) = 5`
     /// - `Baz { a: i32} = 5`
-    /// 
+    ///
     /// In either case this value will be `Some(Literal::i32(5))`
     pub value: Option<Literal>,
     /// The attributes of this variant

Other than that LGTM