rust: macros: update 'paste!' macro to accept string literals

Enable combining identifiers with literals in the 'paste!' macro. This
allows combining user-specified strings with affixes to create
namespaced identifiers.

This sample code:

    macro_rules! m {
        ($name:lit) => {
            paste!(struct [<_some_ $name _struct_>] {})
        }
    }

    m!("foo_bar");

Would previously cause a compilation error. It will now generate:

    struct _some_foo_bar_struct_ {}

Signed-off-by: Trevor Gross <tmgross@umich.edu>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20231118013959.37384-1-tmgross@umich.edu


[ Added `:` before example block. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>