Update string.c

[Cyberguru1]

K&R fast implementation of reverse

[marcluque]

Could you elaborate on why the implementation you are suggesting is faster? 🤔 From my understanding it does the same amount of iterations, but some extra work due to the "reverse indexing".

I put it in compiler explorer with -O3 to see the exact differences, see for yourself: https://godbolt.org/z/1d34bc9f6