common/str.c, include/str.h: introduce `str_concat()` #1714

Job	Run time
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, no)	4m 48s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-1.0)	5m 2s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, no, libusb-0.1)	5m 9s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, no)	4m 46s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-1.0)	5m 13s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, nss, libusb-0.1)	5m 4s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, no)	4m 53s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-1.0)	5m 2s
Analyze (cpp, ubuntu-latest, CC=gcc CXX=g++, openssl, libusb-0.1)	4m 55s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, no)	4m 39s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-1.0)	4m 43s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, no, libusb-0.1)	5m 2s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, no)	4m 43s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-1.0)	4m 52s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, nss, libusb-0.1)	5m 7s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, no)	4m 45s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-1.0)	5m 5s
Analyze (cpp, ubuntu-latest, CC=clang CXX=clang++, openssl, libusb-0.1)	5m 2s
Analyze (python, ubuntu-latest, PYTHON=python3, no, no)	1m 18s
	1h 30m 8s

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