- Temp allocator now supports more than 2 in-flight stacks.

- Printing stacktrace uses its own temp allocator.
- `@pool` no longer takes an argument.
- `Allocator` interface removes `mark` and `reset`.
- DynamicArenaAllocator has changed init function.
- Added `BackedArenaAllocator` which is allocated to a fixed size, then allocates on the backing allocator and supports mark/reset.

lib/std/encoding/json.c3

@@ -23,7 +23,7 @@ fn Object*? parse(Allocator allocator, InStream s)
 	@stack_mem(512; Allocator smem)
 	{
 		JsonContext context = { .last_string = dstring::new_with_capacity(smem, 64), .stream = s, .allocator = allocator };
-		@pool(allocator)
+		@pool()
 		{
 			return parse_any(&context);
 		};