Consider removing implicit/default Layouts

[erik-stripe]

Since an efficient layout is such a crucial part of successfully using Bonsai, it might be better to have folks create their own, rather than providing implicit instances. Look into what it would take to remove the implicits and just use builders.

[tixxit]

I definitely generally agree.

I think the future will likely have people using some sort of Generic-style builder that'll remove a lot of the complexity in constructing instances in the common case (eg Layout.generic[MyLabelType]).

Also, serialization (eg on-disk) is a thing we need to start thinking about. It's unclear to me if we want to conflate in-memory layout with on-disk, but there are some obvious benefits in load time if we do (also, can even imagine an on-disk, memory-mapped back Vec). However, in that case we really need to be sure we don't accidentally swap out the Layout being used, so there can be no generic implicits.