use * for pointer type instead of &

See #770

To help automatically translate code, see the
zig-fmt-pointer-reform-2 branch.

This will convert all & into *. Due to the syntax
ambiguity (which is why we are making this change),
even address-of & will turn into *, so you'll have
to manually fix thes instances. You will be guaranteed
to get compile errors for them - expected 'type', found 'foo'

example/hello_world/hello_libc.zig

@@ -7,7 +7,7 @@ const c = @cImport({
 
 const msg = c"Hello, world!\n";
 
-export fn main(argc: c_int, argv: &&u8) c_int {
+export fn main(argc: c_int, argv: **u8) c_int {
     if (c.printf(msg) != c_int(c.strlen(msg))) return -1;
 
     return 0;