Improve performance of encodeNestedLists (#453)

## Changes
* Closes #46.
* Improve performance of `encodeNestedLists` inside `setSubstitutionSystem$cpp.m` by not using `ReplaceRepeated`.

## Examples
```wl
In[]:= rules = {{{a_, b_}, {a_, c_}, {a_, d_}} :> 
    Module[{$0, $1, $2}, {{$0, $1}, {$1, $2}, {$2, $0}, {$0, $2}, \
{$2, $1}, {$1, $0}, {$0, b}, {$1, c}, {$2, d}, {b, $2}, {d, $0}}]};
AbsoluteTiming[set = SetReplace[{{0, 0}, {0, 0}, {0, 0}}, rules, 100]] // First
AbsoluteTiming[SetReplace[set, rules]][[1]]

Out[]= 0.052426
Out[]= 0.026598
```

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Kernel/setSubstitutionSystem$cpp.m

@@ -77,8 +77,8 @@
    sublists. *)
 (* This format is used to pass both rules and set data into libSetReplace over LibraryLink *)
 
-encodeNestedLists[list_List] :=
-    {list} //. {{l___, List[args___], r___} :> {l, Length[{args}], args, r}}
+encodeNestedLists[list_List] := Flatten @ {Length @ list, encodeNestedLists /@ list}
+encodeNestedLists[arg_] := arg
 
 (* This is the reverse, used to decode set data (a list of expressions) from libSetReplace *)