par_iter can now handle multiple iterators in one_chain #23

Job	Run time
build (BurntSushi/aho-corasick)	2m 38s
build (rust-lang/rust-bindgen)	3m 8s
build (dtolnay/cxx)	2m 56s
build (rust-bakery/nom)	3m 42s
build (sfackler/rust-openssl)	3m 19s
build (serde-rs/serde)	2m 54s
build (clap-rs/clap)	2m 55s
build (tokio-rs/tracing)	3m 3s
build (brendanzab/codespan)	3m 6s
build (rust-lang/regex)	2m 46s
build (dtolnay/syn)	2m 45s
	33m 12s

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