Have node-internal Buffer use JsUint8Array instead of BufferSource
Use of `BufferSource` has too much overhead for the way that
node-internal `Buffer` is used.
Impose limits on facet name length and nesting depth.
Name length limit: 256 chars
Nesting depth limit: 4 (including root)
We could increase these limits if we needed to but it's hard to imagine anyone needing more than this, in fact I have yet to see a use case for multiple levels of child facets at all.