fix(data/list/defs): remove map_head; rename map_last to modify_last (#4507)
fix(data/list/defs): remove map_head; rename map_last to modify_last (#4507)

`map_head` is removed in favour of the equivalent `modify_head`.
`map_last` is renamed to `modify_last` for consistency with the other
`modify_*` functions.




leanprover-community/mathlib,leanprover-community/mathlib