mathlib3

dd6388c4 - chore(analysis/calculus/cont_diff): split into two files (#18627)

chore(analysis/calculus/cont_diff): split into two files (#18627) The file `cont_diff.lean` takes 3824 lines. We cut it into two files, `cont_diff_def.lean` and `cont_diff.lean`, the former (of 1597 lines) containing general definitions and the latter (of 2264 lines) containing results on the preservation of smoothness by the usual operations and various API extensions that depend on these. There is no single change in maths or code, this is purely a split PR.