consider Str, PStrLoc in ElideLists of StackfulHeapIterator (#2075) #14

Job	Run time
build-test (windows-latest, stable, x86_64-pc-windows-msvc)	26m 7s
build-test (macos-11, stable, x86_64-apple-darwin)	13m 45s
build-test (ubuntu-22.04, stable, x86_64-unknown-linux-gnu)	11m 18s
build-test (ubuntu-20.04, stable, x86_64-unknown-linux-gnu, true)	8m 37s
build-test (ubuntu-20.04, stable, i686-unknown-linux-gnu)	14m 1s
build-test (ubuntu-20.04, 1.70, x86_64-unknown-linux-gnu)	6m 49s
build-test (ubuntu-20.04, beta, x86_64-unknown-linux-gnu)	4m 48s
build-test (ubuntu-20.04, nightly, x86_64-unknown-linux-gnu)	5m 25s
logtalk-test	0s
release	0s
	1h 30m 50s

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