Model call chains as an `AbstractTokenTarget` #253

Job	Run time
rustfmt	10s
lint	5s
clippy	10m 24s
test (ubuntu-latest, native)	7m 44s
test (ubuntu-20.04, native)	6m 34s
test (macos-latest, native)	9m 41s
test (ubuntu-20.04, aarch64-unknown-linux-gnu)	4m 16s
	38m 54s

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