Allocate pointers in deserialize

This is a tracking issue for a potential API change: should `deserialize` be passed an allocator?

While this library is designed not to have any allocations beyond the `ArrayList`, some issue arises with pointers. For instance, consider the following structure:

```zig
const data = .{
// The cast is necessary, otherwise the type is interpreted as `const [0..5]u8`.
.name = @as([]const u8, "James"),
.age = @as(u8, 32),
.company = @as([]const u8, "DEV Inc."),
};
```

When serializing it, then deserializing it, one can not pass the following object:\

```zig
var out: @TypeOf(data) = undefined;
```

This is because the compiler will complain that `runtime value contains reference to comptime var` and `note: comptime var pointers are not available at runtime`. The `undefined` will stuff some `0xaaaaaaa` values in the pointers, and this will be considered comptime, and no reference can be taken to this fudge value.

So the current approach is to somehow pre-allocate the pointers, which creates a whole host of other issues (e.g. what size should be allocated?).

The only way around this issue that I see for now, is to add a new `allocator` agument, so that it can be used in this specific instance. Since this is useless most of the time, this could be an optional argument.