wrong_self_convention: For to_* variant don't lint in trait impl taking self when non-Copy type

It relaxes rules for `to_*` variant, so it doesn't lint in trait definitions
and implementations anymore.
Although, non-`Copy` type implementing trait's `to_*` method taking
`self` feels not good (consumes ownership, so should be rather named `into_`), it would be better if this case was a pedantic lint (allow-by-default) instead.

clippy_lints/src/methods/wrong_self_convention.rs

@@ -22,7 +22,7 @@ const CONVENTIONS: [(&[Convention], &[SelfKind]); 9] = [
     // Conversion using `to_` can use borrowed (non-Copy types) or owned (Copy types).
     // Source: https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv
     (&[Convention::StartsWith("to_"), Convention::NotEndsWith("_mut"), Convention::IsSelfTypeCopy(false), 
-    Convention::IsTraitItem(false)], &[SelfKind::Ref]),
+    Convention::IsTraitItem(false), Convention::ImplementsTrait(false)], &[SelfKind::Ref]),
     (&[Convention::StartsWith("to_"), Convention::NotEndsWith("_mut"), Convention::IsSelfTypeCopy(true), 
     Convention::IsTraitItem(false), Convention::ImplementsTrait(false)], &[SelfKind::Value]),
 ];

tests/ui/wrong_self_convention2.rs

@@ -23,7 +23,7 @@ mod issue6983 {
     }
 
     struct FooNoCopy;
-    // trigger lint
+    // don't trigger
     impl ToU64 for FooNoCopy {
         fn to_u64(self) -> u64 {
             2

tests/ui/wrong_self_convention2.stderr

@@ -1,11 +0,0 @@
-error: methods with the following characteristics: (`to_*` and `self` type is not `Copy`) usually take `self` by reference
-  --> $DIR/wrong_self_convention2.rs:28:19
-   |
-LL |         fn to_u64(self) -> u64 {
-   |                   ^^^^
-   |
-   = note: `-D clippy::wrong-self-convention` implied by `-D warnings`
-   = help: consider choosing a less ambiguous name
-
-error: aborting due to previous error
-