Don't use `usingnamespace`

[alichraghi]

There's a single usage of usingnamespace in here

zmath/src/main.zig

Line 10 in f40bb8a

pub usingnamespace @import("zmath.zig");

usingnamespace isn't supported by incremental compilation and is likely going to be removed.
also the possible solutions are pretty trivial. e.g. we can move util.zig content into zmath.zig

[hazeycode]

#14