ENH: add alternative STL ASCII parsers

- In addition to the traditional Flex-based parser, added a Ragel-based
  parser and a handwritten one.

  Some representative timings for reading 5874387 points (1958129 tris):

      Flex   Ragel   Manual
      5.2s   4.8s    6.7s         total reading time
      3.8s   3.4s    5.3s         without point merging

src/OpenFOAM/primitives/strings/string/stringI.H

@@ -222,17 +222,18 @@ inline String Foam::string::validate(const std::string& str)
     String out;
     out.resize(str.size());
 
-    size_type count = 0;
+    size_type len = 0;
     for (auto iter = str.cbegin(); iter != str.cend(); ++iter)
     {
         const char c = *iter;
         if (String::valid(c))
         {
-            out[count++] = c;
+            out[len] = c;
+            ++len;
         }
     }
 
-    out.resize(count);
+    out.resize(len);
 
     return out;
 }