StreamConstPtr: disallow passing a String temporary

[d-a-v]

Unless using it like this:

StreamConstPtr(String("test")).sendAll(elsewhere);

using auto var = StreamConstPtr(String-Temporary) leads to errors because it stores an expected-to-be invalid pointer.

This PR prevents from using temporary String with StreamConstPtr.

(note : StreamConstPtr("test").sendAll(elsewhere) is implicitely using an intermediate String)

edit:

+ missing virtual destructor in Stream::

(warning: deleting object of abstract class type 'Stream' which has non-virtual destructor 
 will cause undefined behavior [-Wdelete-non-virtual-dtor])

[mcspr]

This no longer works

StreamConstPtr test{String("test")}; // String is destroyed next line, can't access it's contents
StreamConstPtr test{"test"}; // implicitly creating String{"test"}, same lifetime as above

This was allowed, but no longer is

StreamConstPtr{"test"}.available(); // ...since we are technically in the same 'expression', still and StreamConstPtr itself is a temporary?

[mcspr]

Not mentioned?

[d-a-v]

commitlog added in OP