A previous change to add std::string_view support to ANTLRInputStream for C++17 caused some trouble because of ABI changes. This has been changed to define 2 constructors, one for std::string_view (for C++17) and the original one for std::string.
This is turn caused an error in ANTLRFileStream, which also takes a string in the constructor (but handling it as file name instead of input). To make this clearer the c-tor taking a std::string has been deleted in ANTLRFileStream and the class now requires to load input via the loadFile() function. This might cause some trouble for those users who had used the std::string constructor of ANTLRFileStream, but I think the better error reporting outweighs the little annoyance.

- Added a default c-tor to the input stream to avoid an ambiquity.
- Changed the input stream API so that it can take a string pointer + length and use that for UTF conversion, avoiding so unnecessary copies. Convenience methods exist to use a std::string or a std::string_view.
- With that only a single load() method is necessary.
- In ANTLRFileStream the other c-tors are now also deleted, as they make no sense there.

C++ is not <any other language where you don't need a useless constructor>