Move narrow() & escape_backslashes_for_json() into common code #833

Job	Run time
codegen	16s
linux-no-asm	3m 28s
linux-32 (Debug)	1m 47s
linux (gcc, g++, Debug, ubuntu-20.04)	2m 47s
chromium	1m 25s
linux-32 (Release)	2m 6s
linux (gcc, g++, Debug, ubuntu-22.04)	3m 2s
linux-32-no-asm	2m 2s
linux (gcc, g++, Release, ubuntu-20.04)	6m 51s
linux (gcc, g++, Release, ubuntu-22.04)	7m 15s
linux (clang, clang++, Debug, ubuntu-20.04)	2m 13s
linux (clang, clang++, Debug, ubuntu-22.04)	2m 46s
linux (clang, clang++, Release, ubuntu-20.04)	4m 11s
linux (clang, clang++, Release, ubuntu-22.04)	4m 51s
mingw	1m 31s
mingw-use-gas	1m 4s
mingw-no-asm	1m 1s
windows_clang (clang, Debug)	3m 21s
windows_clang (clang, Release)	2m 17s
windows_vs (Win32, Debug)	4m 37s
windows_vs-no-asm (Win32)	5m 37s
windows_clang (clang-cl, Debug)	3m 35s
windows_vs (Win32, Release)	4m 55s
mingw-no-asm-explicit	1m 5s
windows_vs-no-asm (x64)	4m 37s
windows_clang (clang-cl, Release)	3m 26s
windows_vs (x64, Debug)	5m 2s
windows_vs (x64, Release)	5m 0s
iOS	36s
Universal Binary Testing (STATIC ON) w/ Ninja	2m 13s
tvOS	51s
mac (Debug, APPLE_STATIC_LOADER=ON)	5m 43s
Universal Binary Testing (STATIC ON) w/ Xcode	2m 8s
mac (Debug, APPLE_STATIC_LOADER=OFF)	3m 27s
Universal Binary Testing (STATIC OFF) w/ Ninja	2m 17s
mac (Release, APPLE_STATIC_LOADER=ON)	3m 9s
Universal Binary Testing (STATIC OFF) w/ Xcode	2m 37s
mac (Release, APPLE_STATIC_LOADER=OFF)	3m 24s
	1h 58m 33s

Provide feedback