Remove the `Font` borrow-bag; put its methods on `FontsImpl` (#8504)

`Font<'a>` was a hand-rolled split borrow of every field of `FontsImpl`,
and it grew a new `&'a mut` for each new feature. `text_layout` now
calls `FontsImpl` directly (`resolve_face`, `glyph_info`,
`allocate_glyph`, `rasterize_cluster`, …) with a cheap `FamilyKey` index
instead of re-hashing the `FontFamily` per glyph.

`font.rs` is renamed to `font_face.rs`, since only `FontFace` is left in
it. `FontsView::characters` replaces `f.fonts.font(family).characters()`
in the font book demo.

`FontsImpl` and `epaint::text::layout` are now crate-private: nothing
outside `epaint` used them, and `Fonts` / `FontsView` cover the public
API. The `fonts` fields of `Fonts` and `FontsView` are private too. The
benchmark that used `layout` directly now calls the new
`Fonts::layout_uncached`.

* [x] I have followed the instructions in the PR template

PR chain, in order:
* #8490
* #8500
* #8501
* #8502
* #8503
* #8504
* #8491
* #8492
* #8493

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Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>