This adds basic custom decoding for the MuPack metadata section of
the incoming JSON/YAML. Because of the type discriminated union nature
of the incoming payload, we cannot rely on the simple built-in JSON/YAML
unmarshaling behavior. Note that for the metadata section -- what is
in this checkin -- we could have, but the IL AST nodes are problematic.
(To know what kind of structure to creat requires inspecting the "kind"
field of the IL.) We will use a reflection-driven walk of the target
structure plus a weakly typed deserialized map[string]interface{}, as
is fairly customary in Go for scenarios like this (though good libaries
seem to be lacking in this area...).
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