mathlib

59874523 - fix(algebra/monoid_algebra/basic): add `int_cast` to `monoid_algebra` instances (#15779)

fix(algebra/monoid_algebra/basic): add `int_cast` to `monoid_algebra` instances (#15779) Note that this reshuffles how some instances are created to ensure that they get the `int_cast` field. Co-authored-by: Eric Rodriguez <37984851+ericrbg@users.noreply.github.com>