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()

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

@@ -68,7 +68,7 @@ inline bool Foam::string::removePath()
         return false;
     }
 
-    this->erase(0, i+1);
+    erase(0, i+1);
     return true;
 }
 
@@ -82,7 +82,7 @@ inline bool Foam::string::removeExt()
         return false;
     }
 
-    this->resize(i);
+    erase(i);
     return true;
 }
 
@@ -163,7 +163,7 @@ inline bool Foam::string::stripInvalid(std::string& str)
             }
         }
 
-        str.resize(nChar);
+        str.erase(nChar);
 
         return true;
     }
@@ -228,7 +228,7 @@ Foam::string::quotemeta(const std::string& str, const char quote)
         sQuoted += c;
     }
 
-    sQuoted.resize(sQuoted.size());
+    sQuoted.shrink_to_fit();
 
     return sQuoted;
 }
@@ -251,7 +251,7 @@ inline String Foam::string::validate(const std::string& str)
         }
     }
 
-    out.resize(len);
+    out.erase(len);
 
     return out;
 }