feat: Implement Chain for tuples up to 12 elements

[poliorcetics]

I think the index + const pattern I used could be reused for other trait implementations, though I
have not checked.

Also, we could do away with done, it's enough to check this.index == $mod_name::LEN in the assert!, but
without benchmarking I'm not sure which is better, especially since adding a single bool is unlikely to be costly
since the futures will probably be much heavier than it.

Helps with #62.

[poliorcetics]

Trivia: this MR is exactly twice the number of the issue it is working on, which is useless but makes me smile :D

[yoshuawuyts]

Thank you!