Improve FlashString portability (#2796)

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`

Sming/Core/FileSystem.h

@@ -235,7 +235,8 @@ template <typename TFileName> inline int fileSetContent(const TFileName& fileNam
 	return fileSystem->setContent(fileName, content, length);
 }
 
-template <typename TFileName, typename TContent> inline int fileSetContent(const TFileName& fileName, TContent content)
+template <typename TFileName, typename TContent>
+inline int fileSetContent(const TFileName& fileName, const TContent& content)
 {
 	CHECK_FS(setContent)
 	return fileSystem->setContent(fileName, content);

docs/source/upgrading/5.1-5.2.rst

@@ -75,3 +75,19 @@ Applications must explicitly call :cpp:func:`HttpRequest::onSslInit` and set the
 This extra step ensures that security checks are not unintentionally bypassed.
 
 The same behaviour is now presented when using Bearssl, and will now fail with ``X509_NOT_TRUSTED``.
+
+
+**FlashString copy support removed**
+
+The :library:`FlashString` 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.