[Tolk] Completely rework stdlib: multiple files and renaming

- split stdlib.tolk into multiple files (tolk-stdlib/ folder)
  (the "core" common.tolk is auto-imported, the rest are
  needed to be explicitly imported like "@stdlib/tvm-dicts.tolk")
- all functions were renamed to long and clear names
- new naming is camelCase

tolk-tester/tests/invalid-pure-3.tolk

@@ -1,12 +1,12 @@
 @pure
 fun validate_input(input: cell): (int, int) {
-    var (x, y, z, correct) = compute_data_size?(input, 10);
+    var (x, y, z, correct) = calculateCellSize(input, 10);
     assert(correct) throw 102;
 }
 
 @pure
 fun someF(): int {
-    var c: cell = begin_cell().end_cell();
+    var c: cell = beginCell().endCell();
     validate_input(c);
     return 0;
 }