COMP: avoid constructor ambiguity for ISpanStream

- compiler cannot decide between std::string and std::string_view
  when creating from 'const char*' without also supplying the size,
  so also supply a 'const char*' constructor.

ENH: additional string_view handling for ITstream and SubStrings

applications/test/stringSplit/Make/files

@@ -1,3 +1,3 @@
-Test-stringSplit.C
+Test-stringSplit.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-stringSplit