EspSoftwareSerial release 6.7.2 fixes warnings with gcc 9.2 (#7133)

* MultiDelegate breaking change and improvements.

The event-multiplexer specialization no longer removes items based on invoke return value.
This is mitigated by new iterators and erase(iterator). Check the documentation comments for
details.

* MultiDelegate: constness on return type warning fix.

* EspSoftwareSerial: Conform to C++ 17 and later.

* EspSoftwareSerial: Resolve g++ 9's c++17 deprecation warnings.

* EspSoftwareSerial, Delegate.h: gcc 9.2 is very strict about template inheritance of ctor and assignment operators, needing a lot

of code duplication, this commit provides that.

* Delegate.h, fix both ambiguous operator= and infinite ctor recursion.

* EspSoftwareSerial release 6.7.2