edit.New -> .NewEditor
Add a static checker, eval.Checker
eval/compile.go -> check.go
eval: Move parsing of special forms into the checker
eval: Implement Checker.checkClosure (oops)
eval/builtin_varset.go: Move some code around
eval: parseVarSetForm -> checkVarSet, report errors better
eval: Remove support for stderr redirection (for now)
eval: Remove a now bogus TODO