we cannot call gc() in execute() because it will gc
anonyous intermediate results which happens when we
construct a list and the elements are calculated by
calling a function thru ocall() which calls execute().
also, the _thiscmd symbol; which is used to keep
a reference to a statement so it wont get garbage
collected; does not work as yyparse() is recursive
(include statements).
we add execrec() function which *only* gets called from
yyparse() when evaluating a statement. it will
keep a stack on the _thiscmd symbol handling the yyparse()
recursion.
we also only call gc() in execrec() before calling
execute(). so execute() will never gc() while evaluating
a statement which prevents the intermediate results
from getting collected.
