Fix HAMT crash when function values used as map keys #58

Job	Run time
test-linux	3m 50s
test-macos	3m 1s
cross-compile (x86_64-macos-none)	3m 46s
cross-compile (aarch64-linux-gnu)	3m 52s
cross-compile (aarch64-macos-none)	3m 41s
cross-compile (x86_64-linux-gnu)	3m 34s
	21m 44s

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