updates for release 0.0.2
tweak release process
types: detect and report cycles in variable expansion

E.g. if someone writes code like

    a = "$b"
    b = "$a"

then we must report that as a programmer error, not go into infinite
recursion.
dag: ensure that variable references are expanded in ActionExpand()

The use case is a bit obscure -- Node objects that aren't in the DAG
-- but not impossible. So it's worth handling correctly.