checkFreed

fn checkFreed(s: *SafeAllocator, footer: *AllocFooter) void {
    if (!s.check_write_after_free) return;
    const memory = footer.userMemory();
    for (memory) |*b| if (b.* != 0x55) {
        panic(
            \\write after free at *{x}
            \\original alloc of {f}: {f}
            \\free: {f}
            \\stack trace:
            // (panic stack trace)
        , .{
            @intFromPtr(b),
            FormatMemory{ .memory = memory, .alignment = footer.userAlign() },
            formatStackTrace(footer.allocTrace(s)),
            formatStackTrace(footer.freeTrace(s)),
        });
    };
}