EvalTrees
The import EvalTree()s have been replaced with Execute +
straight-through code, which let me remove eval_import_state.go.
graphNodeImportStateSub's Execute() function is still using the
old-style (not-refactored) calls to EvalRefresh and EvalWriteState;
those are being saved for a later refactor once all (or at least most)
of the EvalTree()s are gone.
I've added incredibly basic tests for both Execute() functions; they are
only enough to verify basics - the real testing happens in
context_import_test.go.
