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 introduces a new GraphNode, GraphNodeExecutable, which will
gradually replace GraphNodeEvalable as part of the overall removal of
EvalTree()s. Terraform's Graph.walk function will now check if a node is
GraphNodeExecutable and run walker.Execute instead of running through
the EvalTree() and Eval().
For the time being, terraform will panic if a node implements both
GraphNodeExecutable and GraphNodeEvalable. This will be removed when
we've finished removing all GraphNodeEvalable implementations.
The new GraphWalker function, Execute(), is meant to replace both
EnterEvalTree and ExitEvalTree, and wraps the call to the
GraphNodeExecutable's Execute function.
