mathlib3

d2ae4e25 - feat(ring_theory/witt_vector): truncated Witt vectors, definition and ring structure (#5162)

feat(ring_theory/witt_vector): truncated Witt vectors, definition and ring structure (#5162) Co-Authored-By: Rob Y. Lewis <rob.y.lewis@gmail.com> Co-authored-by: Rob Lewis <rob.y.lewis@gmail.com>