Unification-based inference for regions is useful with a language
that has let bindings (which would propagate the regions) and
functions polymorphic over regions. For reasons of simplicity,
ARTIQ has neither, and making unification-based inference work would
essentially mean adding region coercions between most AST nodes,
and having every source subexpression have its own region variable,
with the appropriate subtyping relationship.
It's simpler to just keep that state outside of typedtree.
