[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/a10.tolk

@@ -104,9 +104,9 @@ fun testStartBalanceCodegen2() {
   testDumpDontPolluteStack PROC:<{
     ...
     DUMPSTK
-    x{6d79} PUSHSLICE	//  f s _9
+    x{6d79} PUSHSLICE	//  f s _5
     STRDUMP DROP
-    SBITS	//  f _11
+    SBITS	//  f _6
   }>
 """