[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-2.tolk

@@ -2,7 +2,7 @@ global g: int;
 
 @pure
 fun f_pure(): builder {
-    var b: builder = begin_cell();
+    var b: builder = beginCell();
     g = g + 1;
     return b;
 }