Model call chains as an `AbstractTokenTarget` #255

Job	Run time
clippy	9m 17s
lint	8s
rustfmt	8s
test (ubuntu-latest, native)	6m 18s
test (ubuntu-20.04, native)	7m 19s
test (macos-latest, native)	9m 53s
test (ubuntu-20.04, aarch64-unknown-linux-gnu)	3m 42s
	36m 45s

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