Implement `GetKey` for `KeyMap` #2070

Job	Run time
Embedded	1m 42s
Docs - stable toolchain (recent)	26s
Bench - nightly toolchain (recent)	30s
Docs - nightly toolchain (recent)	38s
Format - nightly toolchain	13s
Integration tests - stable toolchain (26_0)	51s
Lint - nightly toolchain (recent)	36s
Test - 1.63.0 toolchain (minimal)	1m 42s
Integration tests - stable toolchain (25_2)	42s
Test - nightly toolchain (minimal)	20s
Test - 1.63.0 toolchain (recent)	1m 38s
Integration tests - stable toolchain (25_1)	46s
Test - nightly toolchain (recent)	20s
Test - stable toolchain (minimal)	32s
Integration tests - stable toolchain (25_0)	41s
Test - stable toolchain (recent)	21s
Integration tests - stable toolchain (24_2)	47s
Integration tests - stable toolchain (24_1)	41s
Integration tests - stable toolchain (24_0_1)	53s
Integration tests - stable toolchain (23_2)	42s
Integration tests - stable toolchain (23_1)	41s
Integration tests - stable toolchain (23_0)	42s
Integration tests - stable toolchain (22_1)	47s
Integration tests - stable toolchain (22_0)	41s
Integration tests - stable toolchain (0_21_2)	55s
Integration tests - stable toolchain (0_20_2)	48s
Integration tests - stable toolchain (0_19_1)	55s
Integration tests - stable toolchain (0_18_1)	39s
Integration tests - stable toolchain (0_17_1)	43s
	21m 52s

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