Implement `GetKey` for `KeyMap` #2127

Job	Run time
Docs - stable toolchain (recent)	51s
Docs - nightly toolchain (recent)	42s
Bench - nightly toolchain (recent)	3m 40s
Format - nightly toolchain	29s
Embedded	1m 54s
Integration tests - stable toolchain (26_0)	1m 4s
Lint - nightly toolchain (recent)	47s
Integration tests - stable toolchain (25_2)	1m 19s
Test - 1.63.0 toolchain (minimal)	2m 29s
Integration tests - stable toolchain (25_1)	1m 43s
Test - 1.63.0 toolchain (recent)	2m 43s
Test - nightly toolchain (minimal)	1m 18s
Test - stable toolchain (minimal)	2m 16s
Integration tests - stable toolchain (25_0)	57s
Test - nightly toolchain (recent)	1m 27s
Test - stable toolchain (recent)	1m 2s
Integration tests - stable toolchain (24_2)	56s
Integration tests - stable toolchain (24_1)	1m 0s
Integration tests - stable toolchain (24_0_1)	2m 8s
Integration tests - stable toolchain (23_2)	51s
Integration tests - stable toolchain (23_1)	1m 21s
Integration tests - stable toolchain (23_0)	1m 54s
Integration tests - stable toolchain (22_1)	1m 3s
Integration tests - stable toolchain (22_0)	1m 39s
Integration tests - stable toolchain (0_21_2)	1m 57s
Integration tests - stable toolchain (0_20_2)	1m 28s
Integration tests - stable toolchain (0_19_1)	2m 21s
Integration tests - stable toolchain (0_18_1)	1m 29s
Integration tests - stable toolchain (0_17_1)	1m 26s
	44m 14s

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