Add .push_back() and .pop_back() to etl::intrusive_forward_list() #528

Job	Run time
Syntax Check - GCC C++03 Linux No STL (ubuntu-22.04)	16s
Syntax Check - GCC C++03 Linux STL (ubuntu-22.04)	18s
Syntax Check - GCC C++11 Linux No STL (ubuntu-22.04)	26s
Syntax Check - GCC C++11 Linux No STL Force C++03 (ubuntu-22.04)	30s
Syntax Check - GCC C++11 Linux STL (ubuntu-22.04)	28s
Syntax Check - GCC C++11 Linux STL Force C++03 (ubuntu-22.04)	27s
Syntax Check - GCC C++14 Linux STL (ubuntu-22.04)	28s
Syntax Check - GCC C++14 Linux No STL (ubuntu-22.04)	24s
Syntax Check - GCC C++14 Linux No STL Force C++03 (ubuntu-22.04)	24s
Syntax Check - GCC C++14 Linux STL Force C++03 (ubuntu-22.04)	26s
Syntax Check - GCC C++17 Linux No STL (ubuntu-22.04)	29s
Syntax Check - GCC C++17 Linux No STL Force C++03 (ubuntu-22.04)	33s
Syntax Check - GCC C++17 Linux STL (ubuntu-22.04)	34s
Syntax Check - GCC C++17 Linux STL Force C++03 (ubuntu-22.04)	38s
Syntax Check - GCC C++20 Linux STL (ubuntu-22.04)	51s
Syntax Check - GCC C++20 Linux No STL (ubuntu-22.04)	42s
Syntax Check - GCC C++20 Linux No STL Force C++03 (ubuntu-22.04)	41s
Syntax Check - GCC C++20 Linux STL Force C++03 (ubuntu-22.04)	46s
	9m 21s

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