[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
   }>
 """
 

tolk-tester/tests/cells-slices.tolk

@@ -220,11 +220,11 @@ Note, that since 'compute-asm-ltr' became on be default, chaining methods codege
     1 PUSHINT	//  _0 _1=1
     SWAP	//  _1=1 _0
     32 STU	//  _0
-    2 PUSHINT	//  _0 _5=2
-    SWAP	//  _5=2 _0
+    2 PUSHINT	//  _0 _4=2
+    SWAP	//  _4=2 _0
     32 STU	//  _0
-    3 PUSHINT	//  _0 _9=3
-    SWAP	//  _9=3 _0
+    3 PUSHINT	//  _0 _7=3
+    SWAP	//  _7=3 _0
     32 STU	//  _0
   }>
 """

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
   }>
 """
 

tolk-tester/tests/mutate-methods.tolk

@@ -307,7 +307,7 @@ fun main(){}
     ...
     incrementTwoInPlace CALLDICT        //  x y sum1
     -ROT
-    10 PUSHINT  //  sum1 x y _8=10
+    10 PUSHINT  //  sum1 x y _10=10
     incrementTwoInPlace CALLDICT        //  sum1 x y sum2
     s1 s3 s0 XCHG3       //  x y sum1 sum2
   }>
@@ -317,8 +317,8 @@ fun main(){}
 """
  load_next PROC:<{
     //  cs
-    32 LDI      //  _3 cs
-    SWAP        //  cs _3
+    32 LDI      //  _4 cs
+    SWAP        //  cs _4
   }>
 """
 

tolk-tester/tests/null-keyword.tolk

@@ -113,7 +113,7 @@ fun main() {
     CONS	//  numbers
     UNCONS	//  h numbers
     DUP	//  h numbers numbers
-    CAR	//  h numbers _12
+    CAR	//  h numbers _13
 """
 
 @fif_codegen
@@ -133,7 +133,7 @@ fun main() {
 """
   test7 PROC:<{
     ...
-    LDOPTREF	//  b _18 _17
+    LDOPTREF	//  b _8 _7
     DROP	    //  b c
     ISNULL	    //  b _11
     10 MULCONST	//  b _13