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Return a `:create` command that can insert data from a nested hash.

This is limited to `:create` commands only, because automatic restriction
for `:update` commands would be quite complex. It's possible that in the
future support for `:update` commands will be added though.

Another limitation is that it can only work when you're composing
parent and its child(ren), which follows canonical hierarchy from your
database, so that parents are created first, then their PKs are set
as FKs in child tuples. It should be possible to make it work with
both directions (parent => child or child => parent), and it would
require converting input tuples based on how they depend on each other,
which we could do in the future.

Expanding functionality of this method is planned for rom 5.0.

@see Relation#command

@raise NotImplementedError when type is not `:create`

@api public
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