This commit refactors NodeApplyableOutput and NodeDestroyableOutput into
the new Execute() pattern, collapsing the functions in eval_output.go
into one place.
I also reverted a recent decision to have Execute take a _pointer_ to a
walkOperation: I was thinking of interfaces, not constant bytes, so all
it did was cause problems.
And finally I removed eval_lang.go, which was unused.
This commit continues the overall EvalNode removal project.
Something to note: the NodeRefreshableDataResourceInstance's Execute()
function is intentionally refactored in the bare minimum,
hardly-a-refactor style, because we have another ongoing project which
aims to remove NodeRefreshable*s. It is not worth the effort at this
time. We may revisit this decision in the future.
