Fold operation (code/docs/test)

compile.h

@@ -33,6 +33,7 @@ block gen_call(const char* name, block body);
 block gen_subexp(block a);
 block gen_both(block a, block b);
 block gen_collect(block expr);
+block gen_fold(const char* varname, block init, block body);
 block gen_assign(block expr);
 block gen_definedor(block a, block b);
 block gen_condbranch(block iftrue, block iffalse);