chore(data/finsupp/basic): rename type variables (#4624)
Use `M`, `N`, `P` for types with `has_zero`, `add_monoid`, or
`add_comm_monoid` structure, and `R`, `S` for types with at least
a `semiring` instance.
API change: `single_add_erase` and `erase_add_single` now use explicit arguments.
Co-authored-by: Johan Commelin <johan@commelin.net>
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
