Test / Improve tree-shakeability

[gkjohnson]

With #817 the "acceleratedRaycast" function requires storing the original raycast functions, which will cause the three.js classes to not tree-shake.

[gkjohnson]

It may be best to add a "replace" or "register"-style function so the callbacks can be registered in a tree-shakeable way:

replaceRaycastFunction( Mesh.prototype);