Fixed local variable shadowing

Fixed local variable shadowing

In crypto_x25519_inverse(), line 2949 and 2953, we use the ZERO macro in
a context where the enclosing scope already defines the varible `i`.

Turns out ZERO defines `i` for internal use in an enclosed scope. This
trigger a warning in some compilers about variable shadowing: declaring
a local variable with the same name as an enclosing local variable. This
warning is especially annoying when combined with -Werror.

To prevent this, the macros COPY and ZERO have been modified so they use
a variable name that is unlikely to be used anywhere else (`i__`).