Update for zig build changes

- Step now holds the builder and some manifest/cache helpers
- cleaned up proxy names and remaining uses of `std.build`
- removed vicious lie from README

build.zig

@@ -1,6 +1,5 @@
 const std = @import("std");
 const vkgen = @import("generator/index.zig");
-const Step = std.build.Step;
 
 pub const ShaderCompileStep = vkgen.ShaderCompileStep;
 pub const VkGenerateStep = vkgen.VkGenerateStep;