rust: macros: reduce collections in `quote!` macro

rust: macros: reduce collections in `quote!` macro

Remove a handful of unnecessary intermediate vectors and token streams;
mainly the top-level stream can be directly extended with the notable
exception of groups.

Remove an unnecessary `#[allow(dead_code)]` added in commit dbd5058ba60c
("rust: make pin-init its own crate").

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>