Move UnresolvedVariables into scope, constify
Remove ScopeNew

Previously, scopes were created explicitly using ScopeNew before use.
Now, attempting to add a variable or set a current scope will implicitly
create a new scope.
Rename ScopeDelete -> ScopeClear, to clarify updated semantics
Merge pull request #422 from sigurdteigen/cleanup/implicit-scope

Cleanup/implicit scope