Changed how Allocator work to be more Zig like

Now GpuArena doesn't do everything.

- Created GpuDeviceAllocator that allocate to the device
- GpuArena become GpuArenaAllocator and use a child_allocator like
std.heap.ArenaAllocator

src/lib.zig

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