Refactor evaluation to not be handled in a single large class

UpsilonTests/GeneralTests/IfTests.cs

@@ -31,9 +31,9 @@ return false";
             var input  =
 $@"
 if {condition} then
-    val = {in1}
+    return {in1}
 else
-    val = {in2}
+    return {in2}
 end";
             var actual = Executor.EvaluateScript<long>(input, Options);
             Assert.Equal(expected, actual);
@@ -47,11 +47,11 @@ end";
             var input =
                 $@"
 if {condition1} then
-    val = {in1}
+    return {in1}
 elseif {condition2} then
-    val = {in2}
+    return {in2}
 else
-    val = {in3}
+    return {in3}
 end";
             var actual = Executor.EvaluateScript<long>(input, Options);
             Assert.Equal(expected, actual);