It makes the code a little nicer to read.

The `&[ast::Variant]` field isn't used.

The `Vec<Ident>` field is only used for its length, but that's always
the same as the length of the `&[Ident]` and so isn't necessary.

It's unused.

They are always `false`.

It's unused.

This was accidentally left behind in a previous commit.

Improve a comment, and panic on an impossible code path.

The existing derive code allows for various possibilities that aren't
needed in practice, which complicates the code. There are only a few
auto-derived traits and new ones are unlikely, so this commit simplifies
things.

- `PtrTy` has been eliminated. The `Raw` variant was never used, and the
  lifetime for the `Borrowed` variant was always `None`. That left just
  the mutability field, which has been inlined as necessary.
- `MethodDef::explicit_self` was a confusing `Option<Option<PtrTy>>`.
  Indicating either `&self` or nothing. It's now a `bool`.
- `borrowed_self` is renamed as `self_ref`.
- `Ty::Ptr` is renamed to `Ty::Ref`.

It's unused.

Because a `Literal` is a type of expression, and is simply the wrong
name for this.

Because that's all that is needed in practice.