Current tuple macro will correctly drop initialized fields when an error is encountered. But, it does not use a drop guard like the derive macros currently do, so it's not panic safe.

Generating correct drop guard logic with macro_rules gets exceptionally ugly, and thus hard to understand and maintain. This PR rewrites the current tuple macro into a function that generates a TokenStream (like a proc-macro) so that the implementation is much easier to reason about (doesn't require multiple recursing declarative macros), in addition to using drop guard like the derive macros so that we're panic safe.

I've also bundled this into a build step so that we don't require a separate crate to expose a proc-macro, which would have to be independently versioned and published.