Improve FlashString portability

[mikee47]

This PR removes copy support from the FlashString library. Doing so simplifies the code, improves performance and portability to other compilers (e.g. clang).

The FlashString library previously supported copies (references) like this::

 FlashString emptyString;
 FlashString stringCopy(FS("Inline string"));

 DEFINE_FSTR_DATA_LOCAL(flashHelloData, "Hello");
 auto myCopy = flashHelloData;

These will now fail to compile.
Copy construction and assignment has been explicitly deleted so avoid unintentional side-effects.
Objects should always be passed by reference.

This change has the additional benefit of catching pass-by-copy errors. These function/method templates have been fixed:

• fileSetContent
• IFS::File::setAttribute
• IFS::FileSystem::setUserAttribute
• IFS::ArchiveStream::setUserAttribute

[slaff]

@mikee47 can you add this information also to the UPGRADING document?

[mikee47]

@mikee47 can you add this information also to the UPGRADING document?

? Already there...