Implement `GetKey` for `KeyMap` #2128

Job	Run time
Test - 1.63.0 toolchain (minimal)	26m 4s
Test - nightly toolchain (minimal)	14m 8s
Test - 1.63.0 toolchain (recent)	25m 50s
Test - stable toolchain (minimal)	17m 41s
Test - nightly toolchain (recent)	14m 54s
Test - stable toolchain (recent)	18m 37s
Bench - nightly toolchain (recent)	3m 11s
Docs - stable toolchain (recent)	1m 33s
Docs - nightly toolchain (recent)	1m 11s
Embedded	1m 54s
Format - nightly toolchain	20s
Integration tests - stable toolchain (26_0)	1m 7s
Lint - nightly toolchain (recent)	1m 20s
Integration tests - stable toolchain (25_2)	1m 23s
Integration tests - stable toolchain (25_1)	1m 21s
Integration tests - stable toolchain (25_0)	48s
Integration tests - stable toolchain (24_2)	1m 1s
Integration tests - stable toolchain (24_1)	1m 22s
Integration tests - stable toolchain (24_0_1)	1m 28s
Integration tests - stable toolchain (23_2)	50s
Integration tests - stable toolchain (23_1)	1m 19s
Integration tests - stable toolchain (23_0)	1m 25s
Integration tests - stable toolchain (22_1)	1m 14s
Integration tests - stable toolchain (22_0)	1m 0s
Integration tests - stable toolchain (0_21_2)	1m 12s
Integration tests - stable toolchain (0_20_2)	1m 10s
Integration tests - stable toolchain (0_19_1)	1m 42s
Integration tests - stable toolchain (0_18_1)	1m 43s
Integration tests - stable toolchain (0_17_1)	1m 13s
	2h 28m 1s

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