mathlib3

3c006126 - feat(data/{seq,stream}): cons_injective2 (#15832)

feat(data/{seq,stream}): cons_injective2 (#15832) Also some simple API lemmas for `nth` I didn't change the style of the `stream/init` file. Co-authored-by: Yakov Pechersky <ypechersky@treeline.bio>