Optimise desugaring of let rec's with a single binding
Optimise desugaring of let rec's with a single binding

bixuanzju/fcore,zhiyuanshi/fcore,zhiyuanshi/fcore,wxzh/fcore,wxzh/fcore,xnning/fcore,bixuanzju/fcore,hkuplg/fcore,bixuanzju/fcore,wxzh/fcore,xnning/fcore,hkuplg/fcore,xnning/fcore,hkuplg/fcore,zhiyuanshi/fcore