mach: eliminate heap allocation of Core

Prior to this change `mach.Core.init` would heap allocate the structure, returning `*Core`:

```zig
app.core = try mach.Core.init(allocator, .{});
```

This was obviously not ideal, but wasn't possible to eliminate before due to how Core was
entangled with the platform abstraction. Now that it has been removed, we can reduce Core
initialization to take a `*Core` to initialize. In practice this means initialization looks
something like this:

```zig
try mach.Core.init(&app.core, alloctor, .{});
```

Or more simply:

```zig
try app.core.init(allocator, .{});
```

And we eliminate the `*Core` allocation entirely in most cases.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/engine.zig

@@ -25,15 +25,16 @@ pub fn App(
 
     return struct {
         engine: ecs.World(modules),
+        core: Core,
 
         pub fn init(app: *@This()) !void {
+            try app.core.init(allocator, .{});
             app.* = .{
+                .core = app.core,
                 .engine = try ecs.World(modules).init(allocator),
             };
-            var core = try allocator.create(Core);
-            core.* = try Core.init(allocator, .{});
-            app.engine.set(.mach, .core, core);
-            app.engine.set(.mach, .device, core.device());
+            app.engine.set(.mach, .core, &app.core);
+            app.engine.set(.mach, .device, app.core.device());
             try app_init(&app.engine);
         }