fix!: consume BeatmapAttributesBuilder on build

[MaxOhn]

Turns out #22 was only a band-aid for an underlying issue.

Passing a Beatmap to BeatmapAttributesBuilder would make the builder hold onto it even after build is called, meaning the map would still be marked as used. This is because build does not consume the builder to allow for re-use.

This PR makes build take ownership of the builder, causing future interactions with that same builder instance to fail with "null pointer passed to rust". Since ownership is taken, returning from build will drop self and all its fields, including the beatmap, causing it to no longer be marked as used.

Until a version containing this fix is released (which will be v2.0.0), there is an available workaround:

• Creating a BeatmapAttributesBuilder and storing it in a variable
• Calling .build() on that variable
• Explicitly freeing the builder.

const builder = new rosu.BeatmapAttributesBuilder({
  map: my_beatmap,
  mods: my_mods,
});
const attributes = builder.build();
builder.free(); // <-

// now map can be free'd too
my_beatmap.free();