Store index of slot in the pool instead of a pointer or a distance #755

Job	Run time
Lint	34s
GCC (5)	4m 21s
GCC (6)	3m 27s
GCC (7, -fsanitize=leak -fno-sanitize-recover=all)	3m 49s
GCC (8, -fsanitize=undefined -fno-sanitize-recover=all)	5m 44s
GCC (9, -fsanitize=address -fno-sanitize-recover=all)	4m 9s
GCC (10, -funsigned-char)	3m 22s
GCC (11)	3m 53s
Clang (3.8, -stdlib=libc++)	3m 40s
Clang (3.9, -stdlib=libc++)	4m 20s
Clang (4.0, -stdlib=libc++)	5m 7s
Clang (5.0)	4m 27s
Clang (6.0)	4m 30s
Clang (7)	3m 10s
Clang (8, -fsanitize=leak -fno-sanitize-recover=all)	4m 59s
Clang (9, -fsanitize=undefined -fno-sanitize-recover=all)	3m 39s
Clang (10, -fsanitize=address -fno-sanitize-recover=all)	5m 15s
Clang-Tidy	6m 3s
XCode (11.7)	5m 31s
XCode (12.4)	5m 57s
XCode (13.2.1)	5m 34s
Arduino (arduino:avr, arduino:avr:uno)	43s
Arduino (arduino:samd, arduino:samd:mkr1000)	1m 9s
PlatformIO (atmelavr, leonardo, SD, Ethernet, avr)	37s
PlatformIO (espressif8266, huzzah, esp8266)	3m 38s
PlatformIO (espressif32, esp32dev, Ethernet, esp8266)	2m 39s
PlatformIO (atmelsam, mkr1000USB, SD, Ethernet, esp8266)	1m 1s
PlatformIO (teensy, teensy31, esp8266)	1m 32s
PlatformIO (ststm32, adafruit_feather_f405, SD, Ethernet, esp8266)	3m 36s
PlatformIO (nordicnrf52, adafruit_feather_nrf52840, SD, Ethernet, esp8266)	2m 11s
Test configuration on Windows	24s
Test configuration on Linux	44s
Particle (argon)	33s
Amalgamate ArduinoJson.h	6s
GCC for ARM processor	4m 0s
Coverage	3m 34s
Valgrind	3m 39s
Amalgamate ArduinoJson.hpp	7s
CodeQL	13m 48s
ESP-IDF	2m 16s
	2h 17m 48s

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