[Tolk] Refactor: get rid of split_vars, construct valid LET ops

In FunC (and in Tolk before), tensor vars (actually occupying
several stack slots) were represented as a single var in terms
or IR vars (Ops):
> var a = (1, 2);
> LET (_i) = (_1, _2)

Now, every tensor of N stack slots is represented as N IR vars.
> LET (_i, _j) = (_1, _2)

This will give an ability to control access to parts of a tensor
when implementing `tensorVar.0` syntax.

tolk-tester/tests/logical-operators.tolk

@@ -332,15 +332,15 @@ These are moments of future optimizations. For now, it's more than enough.
     DUP	//  x x
     IFNOTJMP:<{	//  x
       DROP	//
-      1 PUSHINT	//  _7=1
+      1 PUSHINT	//  _5=1
     }>	//  x
     DUP	//  x x
     IFNOTJMP:<{	//  x
       DROP	//
-      1 PUSHINT	//  _8=1
+      1 PUSHINT	//  _6=1
     }>	//  x
     100 THROWIFNOT
-    -4 PUSHINT	//  _12=-4
+    -4 PUSHINT	//  _9=-4
   }>
 """