set `#![deny(clippy::arithmetic_side_effects)]` in font_types

At the very least in the `fixed` module, but let's review the whole crate.

> From working on #970 it seems that the clippy deny directive does not necessarily capture overflows for our `fixed_impl` types. When I add that directive, the fvar normalization still compiles and the directive does not capture the potential overflow.

_Originally posted by @drott in https://github.com/googlefonts/fontations/issues/967#issuecomment-2188339977_

Begin in the fixed module and inspect the fixed_impl types involved in fvar normalization. Review the whole font_types crate for the lint, then confirm compilation and that the directive catches the intended arithmetic overflow cases.

Written by the indexing model from the issue text.