Model call chains as an `AbstractTokenTarget` #245

Job	Run time
rustfmt	11s
lint	12s
clippy	10m 24s
test (ubuntu-latest, native)	6m 31s
test (ubuntu-20.04, native)	6m 19s
test (macos-latest, native)	9m 46s
test (ubuntu-20.04, aarch64-unknown-linux-gnu)	4m 33s
	37m 56s

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