This commit consist a series of fixes to make the reach the main
goal of the commit:
* Fix the 'MakeNodeFromExprs' behavior so it returns a pointer
to a node and if the number of input exprs is zero then nil.
* Fix all the eval loop to return immediately in case of any error
duh!
* Add a `errtype` field to `Error` with an `ErrType` enum type
that indicates the type of the error, the type being syntax,
semantic and runtime error at the moment.
* Rename some of the Error functions to match the error type. For
Exapmle `MakeSyntaxError` and `MakeSemanticError`
Implement a new algorithm to eliminate TCO while preventing
premature execution exit by rewriting the evaluation tree and
replacing old nodes with new set of forms, for example the result of
some old form's evaluation, a body of a function or anything in
that nature. Couple the new forms with a possible new execution scope.
Sameer Rahmani
Mary Salehi