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See previous commit for more discussion of the child packages to come,
or the schema.md file in the docs dir, which describes similar.

Several large todos in comments.

Quite importantly: typed.Node is now an **interface**, not a concrete
struct itself.  The godoc comment should explain this nicely; long
story short, this is going to be relevant when we get to codegen and
other advanced form of native integration.
Signed-off-by: Eric Myhre <hash@exultant.us>