remove using namepace

[KarthikKaravatt]

This removes the use of usingnamespace #7 but the import is done a little bit differently as seen in the updated readme. This seems like the simplest way to fix this, in terms of changes made.

This is done because usingnamesapce is going to be removed soon and also without it we can use incremental compilation.

[OndraVoves]

It might be better to move the code from zmath.zig directly to main.zig. It will be without usingnamespace and at the same time backwards compatible.

[hazeycode]

As @OndraVoves pointed out we can just move the contents of zmath.zig to the root module file.

[hazeycode]

I also moved util to avoid the circular import.