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

src/meshTools/edgeMesh/edgeMeshFormats/obj/OBJedgeFormat.C

@@ -116,10 +116,9 @@ bool Foam::fileFormats::OBJedgeFormat::read(const fileName& filename)
     {
         string line = this->getLineNoComment(is);
 
-        // handle continuations
-        if (line.back() == '\\')
+        // Handle continuations
+        if (line.removeEnd("\\"))
         {
-            line.resize(line.size()-1);
             line += this->getLineNoComment(is);
         }