patch for speed

[paul zimmermann]

> I'd like to understand the benefits of ./speed mpz_legendre first, since
> I don't really see any. Currently, it's an alias for mpz_jacobi, and I'm
> not aware of any concrete plans to change that.

the benefit for the user is to be able to measure the speed of mpz_legendre.
Since the documentation does not say that mpz_legendre is an alias for
mpz_jacobi, I believe it is legitimate.

Paul