charmap().mappings() and Cmap::from_mappings have different types

Boy this took me too long to work out: `write_fonts::tables::Cmap::from_mappings(font.charmap().mappings().chain(...))` doesn't work, because:

```rust
pub fn from_mappings(
mappings: impl IntoIterator,
)
```

but

```rust
impl Iterator for Mappings<'_> {
type Item = (u32, GlyphId);
```

They're different crates so there's no reason to expect that they should be the same, but the (lack of) parallelism is confusing.

Start by comparing the `Mappings` iterator implementation with `Cmap::from_mappings`, focusing on their `Item` types and the `charmap().mappings()` call shown in the issue. Determine the intended compatible API relationship; done means the confusing type mismatch is resolved or its required conversion is clearly established.

Written by the indexing model from the issue text.