SWIG pickling improvements (and other cleanup) (#6133)

* wip

* - avoid leaking memory after instantiating UChar_Vect
- fix some indentation
- make it easier to read/write pickles as native byte arrays from Java and C#
- add tests

---------

Co-authored-by: Tosco, Paolo <paolo.tosco@novartis.com>

Code/JavaWrappers/csharp_wrapper/RDKitCSharpTest/MolToFromByteArray.cs

@@ -0,0 +1,30 @@
+// Linux:
+// compile with
+// mcs -platform:x64 -r:../RDKit2DotNet.dll -out:MolToFromByteArray.exe  MolToFromByteArray.cs
+// and run with
+// LD_LIBRARY_PATH=..:$RDBASE/lib:$LD_LIBRARY_PATH MONO_PATH=.. mono MolToFromByteArray.exe
+
+using System.IO;
+using System.Diagnostics;
+using GraphMolWrap;
+
+public class MolToFromByteArrayTest
+{
+    static void Main(string[] args)
+    { 
+		string smi = "CN(C)c1ccc2c(=O)cc[nH]c2c1";
+		string pklFileName = "quinolone.pkl";
+		{
+			ROMol mol = RWMol.MolFromSmiles(smi);
+			byte[] pkl = mol.ToByteArray();
+            File.WriteAllBytes(pklFileName, pkl);
+			mol.Dispose();
+		}
+		{
+			byte[] pkl = File.ReadAllBytes(pklFileName);
+			ROMol mol = ROMol.FromByteArray(pkl);
+			Debug.Assert(mol.MolToSmiles() == smi);
+			mol.Dispose();
+		}
+   }
+}