fix: replace_derive_with_manual_impl drops #[cfg] on Debug (and other traits)

[xunilrj]

This PR fixes a problem when struct/enum fields/variants have #[cfg(...)] attributes. In theses cases replace_derive_with_manual_impl access the field/variant directly without the attribute. Example:

  struct Foo {
      bar: String,
      #[cfg(feature = "baz")]
      baz: u32,
  }

  impl Debug for Foo {
      fn fmt(&self, f: &mut Formatter<'_>) -> Result {
          f.debug_struct("Foo").field("bar", &self.bar).field("baz", &self.baz).finish()
      }
  }

One case were I was not able to fix is the case of #[cfg(...)] inside tuple structs. Unfortunately rustc does not accept #[cfg(...)] when pattern matching:

match self {  
      //            v `rustc` complains here
      Self::V(arg0, #[cfg(feature = "x")] arg1) => { ... } 
  }

That would demand something like the below, which becomes very complex, very quickly. I think makes more sense to remove this limitation in rustc directly. Later, I will see if am able to do this.

match self {
      #[cfg(feature = "x")]
      Self::V(arg0, arg1) => { .. }
      #[cfg(not(feature = "x"))]
      Self::V(arg0) => {  ... }
  }

The same problem happens for other traits: Clone, Default, Hash, PartialEq, PartialOrd. If this PR is merged I can create the next ones, as they are very similar.

Another case is #[cfg_attr] which is very similar but it is much harder to deal with. Not sure if we want to support it or not.

One problem I had was that I was not able to find an easy way to create statements with attributes in one go, something like make::stmt_with_attrs(...). That made the code more complex than necessary as I first create blocks, and then splice statements with their attributes.