Revert "improve abstraction for AssocList, fix bug in optional else (#315)" (#320)

This reverts commit bf6bb800c4.

executable_semantics/testdata/zero.golden

@@ -1 +1,72 @@
+********** source program **********
+fn main () -> Int {
+return 0;
+
+}
+********** type checking **********
+--- step exp Int --->
+--- step exp Int --->
+
+********** type checking complete **********
+fn main () -> Int {
+return 0;
+}
+********** starting execution **********
+********** initializing globals **********
+--- step exp () --->
+********** calling main function **********
+{
+stack: top{main()<-1>}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- step exp main() --->
+{
+stack: top{main<-1> :: main()<0>}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- step exp main --->
+{
+stack: top{fun<main><-1> :: main()<0>}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- handle value fun<main> with main()<1>(fun<main>,) --->
+{
+stack: top{()<-1> :: main()<1>(fun<main>,)}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- step exp () --->
+{
+stack: top{()<-1> :: main()<1>(fun<main>,)}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- handle value () with main()<2>(fun<main>,(),) --->
+pattern_match((), ())
+{
+stack: main{return 0;<-1>} :: top{}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- step stmt return 0; --->
+{
+stack: main{0<-1> :: return 0;<0>} :: top{}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- step exp 0 --->
+{
+stack: main{0<-1> :: return 0;<0>} :: top{}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
+--- handle value 0 with return 0;<1>(0,) --->
+{
+stack: top{0<-1>}
+heap: fun<main>, 
+env: main: fun<main>, 
+}
 result: 0