Map<ShapeId, Trait> getTraits() {
        return traits.get();
    }
Removes mixins from a shape and flattens them into the shape.

<p>Flattening a mixin into a shape copies the traits and members of a
mixin onto the shape, effectively resulting in the same shape but with
no trace of the mixin relationship.

@return Returns the updated builder.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/AbstractShapeBuilder.java