ENH: improve HashTable iterator access and management

- provide key_iterator/const_key_iterator for all hashes,
  reuse directly for HashSet as iterator/const_iterator, respectively.

- additional keys() method for HashTable that returns a wrapped to
  a pair of begin/end const_iterators with additional size/empty
  information that allows these to be used directly by anything else
  expecting things with begin/end/size. Unfortunately does not yet
  work with std::distance().

  Example,
     for (auto& k : labelHashTable.keys())
     {
        ...
     }

applications/test/cplusplus1/Make/files

@@ -0,0 +1,3 @@
+Test-cpluplus1.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-cpluplus1

applications/test/cplusplus1/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = -I$(LIB_SRC)/cfdTools/include */
+/* EXE_LIBS = -lfiniteVolume */

applications/test/cplusplus1/Test-cpluplus1.C

@@ -0,0 +1,88 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Copyright (C) 2017 OpenCFD Ltd.
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
+    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Description
+    Test miscellaneous C++ templates/functionality.
+
+\*---------------------------------------------------------------------------*/
+
+#include "string.H"
+#include "IOstreams.H"
+#include "UList.H"
+#include "HashSet.H"
+
+#include <typeinfo>
+#include <type_traits>
+#include <utility>
+
+using namespace Foam;
+
+// Macros to stringify macro contents.
+#define STRINGIFY(content)      #content
+#define STRING_QUOTE(input)     STRINGIFY(input)
+
+#define PRINT_TYPEID(arg)       \
+    Info<< typeid(arg).name() << " <= typeid of " << STRING_QUOTE(arg) << nl
+
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+// Main program:
+
+int main(int argc, char *argv[])
+{
+    Info<< "various declaration types" << nl << nl;
+
+    PRINT_TYPEID(label);
+    PRINT_TYPEID(decltype(UList<label>::value_type()));
+    PRINT_TYPEID(decltype(std::declval<UList<label>>().cbegin()));
+    PRINT_TYPEID(decltype(*(std::declval<UList<label>>().cbegin())));
+    Info<< nl;
+
+    PRINT_TYPEID(decltype(HashTable<label>::key_type()));
+    PRINT_TYPEID(decltype(HashTable<label>::value_type()));
+    // Not yet: PRINT_TYPEID(decltype(HashTable<label>::mapped_type()));
+    PRINT_TYPEID(decltype(std::declval<HashTable<label>>().begin()));
+    PRINT_TYPEID(decltype(std::declval<const HashTable<label>>().begin()));
+    PRINT_TYPEID(decltype(*(std::declval<HashTable<label>>().begin())));
+    PRINT_TYPEID(decltype(*(std::declval<const HashTable<label>>().begin())));
+
+    PRINT_TYPEID(decltype(std::declval<const HashTable<label>>().keys()));
+    Info<< nl;
+
+    PRINT_TYPEID(decltype(HashSet<label>::key_type()));
+    PRINT_TYPEID(decltype(HashSet<label>::value_type()));
+    // Not yet: PRINT_TYPEID(decltype(HashSet<label>::mapped_type()));
+    PRINT_TYPEID(decltype(std::declval<HashSet<label>>().begin()));
+    PRINT_TYPEID(decltype(std::declval<const HashSet<label>>().begin()));
+    PRINT_TYPEID(decltype(*(std::declval<HashSet<label>>().begin())));
+    PRINT_TYPEID(decltype(*(std::declval<const HashSet<label>>().begin())));
+    Info<< nl;
+
+    Info << "\nEnd\n" << endl;
+
+    return 0;
+}
+
+
+// ************************************************************************* //