ENH: rationalize some string methods.

- silently deprecate 'startsWith', 'endsWith' methods
  (added in 2016: 2b14360662), in favour of
  'starts_with', 'ends_with' methods, corresponding to C++20 and
  allowing us to cull then in a few years.

- handle single character versions of starts_with, ends_with.

- add single character version of removeEnd and silently deprecate
  removeTrailing which did the same thing.

- drop the const versions of removeRepeated, removeTrailing.
  Unused and with potential confusion.

STYLE: use shrink_to_fit(), erase()

applications/utilities/mesh/manipulation/objToVTK/objToVTK.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
+    Copyright (C) 2019 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,7 +52,7 @@ string getLine(std::ifstream& is)
     {
         std::getline(is, line);
     }
-    while (line.size() && line[0] == '#');
+    while (line.starts_with('#'));
 
     return line;
 }