Changed GpuAllocator to be like std.mem.Allocator

Now it is 2 ptr and I created a GpuArena. Point is to be like Zig, a
const allocator and a var arena that track everything.

src/lib.zig

@@ -1,4 +1,5 @@
 pub const GpuAllocator = @import("GpuAllocator.zig");
+pub const GpuArena = @import("GpuArena.zig");
 pub const GpuBuffer = @import("GpuBuffer.zig");
 pub const GpuDevice = @import("GpuDevice.zig");
 pub const GpuPipeline = @import("GpuPipeline.zig");