Add Directory method

[b5]

casters need to be able to add a "directory" of byte slices. Maybe:

type Node struct {
   Name string
   Data []byte
   Children []*Node
}

type castore interface {
   ...
   AddTree(node *Node) error
}

from there we could add helper methods that convert things like .zip archives & unix directories into a tree of nodes.

[b5]

for the IPFS implementation we'd have to artificially create a files object to add from.

[b5]

should have structured recent commits as a PR, this is now closed with a recent refactor.