eval: Fix StreamType todos

* Write fdStream explicitly instead of using 0 value;
* Fix "use 0 for closure stream types"
eval: unusedStream is now 0
eval: print Closure without the &
eval: builtin.go -> builtin-func.go; builtin_varset.go -> builtin-special.go
eval/builtin-special.go: Comments
eval: Add new builtin special form: del