Fixes foldl() and foldr() + foldl() with start parameter.
Fixes foldl() and foldr() + foldl() with start parameter.

This fixes the (potential) multi-evaluation of the sequence parameter in
foldl() and foldr().

It also adds a foldl() version which gets a start parameter. This allows
for creating a result with a different type than the elements of the
sequence.

zachaysan/Nim,jsanjuas/Nim,singularperturbation/Nim,mbaulch/Nim,zachaysan/Nim,mbaulch/Nim,russpowers/Nim,singularperturbation/Nim,dom96/Nim,singularperturbation/Nim,dom96/Nim,dom96/Nim,mbaulch/Nim,singularperturbation/Nim,zachaysan/Nim,russpowers/Nim,jfhg/Nim,jsanjuas/Nim,tulayang/Nim,endragor/Nim,jfhg/Nim,haiodo/Nim,dom96/Nim,russpowers/Nim,zachaysan/Nim,mbaulch/Nim,russpowers/Nim,endragor/Nim,haiodo/Nim,mbaulch/Nim,tulayang/Nim,jfhg/Nim,dom96/Nim,mbaulch/Nim,endragor/Nim,dom96/Nim,jfhg/Nim,singularperturbation/Nim,jsanjuas/Nim,endragor/Nim,haiodo/Nim,russpowers/Nim,jsanjuas/Nim,endragor/Nim,mbaulch/Nim,zachaysan/Nim,russpowers/Nim,endragor/Nim,endragor/Nim,jfhg/Nim,tulayang/Nim,tulayang/Nim,russpowers/Nim,tulayang/Nim,jsanjuas/Nim,haiodo/Nim,tulayang/Nim,haiodo/Nim,zachaysan/Nim,jsanjuas/Nim,haiodo/Nim,haiodo/Nim,singularperturbation/Nim,singularperturbation/Nim,dom96/Nim,jfhg/Nim,zachaysan/Nim,jfhg/Nim,jsanjuas/Nim