ENH: consolidate, cleanup some string methods

- consolidate word::validated() into word::validate() and also allow
  as short form for string::validate<word>(). Also less confusing than
  having similarly named methods that essentially do the same thing.

- more consistent const access when iterating over strings

- add valid(char) for keyType and wordRe

applications/utilities/mesh/conversion/ccm/foamToCcm/foamToCcm.C

@@ -207,7 +207,7 @@ int main(int argc, char *argv[])
         // #include "checkHasMovingMesh.H"
         // #include "checkHasLagrangian.H"
 
-        IOobjectList objects(mesh, timeDirs[timeDirs.size()-1].name());
+        IOobjectList objects(mesh, timeDirs.last().name());
 
         forAll(timeDirs, timeI)
         {

applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L

@@ -488,7 +488,7 @@ mtype                 {space}"MTYPE:"{space}
 
 <cellStreamTitle>{spaceNl}{word}{spaceNl} {
 
-        word streamName(Foam::string::validate<word>(YYText()));
+        const word streamName(word::validate(YYText()));
 
         BEGIN(cellStreamFlags);
     }

applications/utilities/mesh/conversion/gmshToFoam/gmshToFoam.C

@@ -370,7 +370,7 @@ void readPhysNames(IFstream& inFile, Map<word>& physicalNames)
             lineStr >> regionI >> regionName;
 
             Info<< "    " << regionI << '\t'
-                << string::validate<word>(regionName) << endl;
+                << word::validate(regionName) << endl;
         }
         else if (nSpaces == 2)
         {
@@ -380,21 +380,21 @@ void readPhysNames(IFstream& inFile, Map<word>& physicalNames)
             if (physType == 1)
             {
                 Info<< "    " << "Line " << regionI << '\t'
-                    << string::validate<word>(regionName) << endl;
+                    << word::validate(regionName) << endl;
             }
             else if (physType == 2)
             {
                 Info<< "    " << "Surface " << regionI << '\t'
-                    << string::validate<word>(regionName) << endl;
+                    << word::validate(regionName) << endl;
             }
             else if (physType == 3)
             {
                 Info<< "    " << "Volume " << regionI << '\t'
-                    << string::validate<word>(regionName) << endl;
+                    << word::validate(regionName) << endl;
             }
         }
 
-        physicalNames.insert(regionI, string::validate<word>(regionName));
+        physicalNames.insert(regionI, word::validate(regionName));
     }
 
     inFile.getLine(line);

applications/utilities/mesh/conversion/ideasUnvToFoam/ideasUnvToFoam.C

@@ -512,7 +512,7 @@ void readSets
             >> dofSet >> tempSet >> contactSet >> nFaces;
 
         is.getLine(line);
-        word groupName = string::validate<word>(line);
+        const word groupName = word::validate(line);
 
         Info<< "For group " << group
             << " named " << groupName