Add `err2str` mirroring `av_err2str` (#137) #372

Job	Run time
rust_clippy_check_windows	1m 51s
build_dynamic_and_test_windows_pre	6m 9s
rust_clippy_check_ubuntu	5m 0s
build_static_and_test_ubuntu (release/6.0, nightly, no valgrind)	5m 16s
rustfmt_check (macos-latest)	20s
build_static_and_test_ubuntu (release/6.0, 1.70.0, no valgrind)	5m 21s
rustfmt_check (windows-latest)	41s
docs_rs_check	1m 6s
build_static_and_test_ubuntu (release/6.1, nightly, valgrind)	1h 3m 37s
rustfmt_check (ubuntu-latest)	12s
build_static_and_test_ubuntu (release/6.1, nightly, no valgrind)	6m 12s
build_static_and_test_ubuntu (release/6.1, 1.70.0, no valgrind)	5m 40s
build_dynamic_and_test_windows (nightly)	2m 37s
build_dynamic_and_test_windows (1.70.0)	2m 47s
	1h 46m 49s

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