Export for GlyphExtents and UnicodeBuffer internals

Hello

I am trying to redo the wasm shaper for HarfRust but as an external crate on top of it and `skrifa`. I am still on the phase of trying to port the code and I have not kept up with the changes from rustybuzz to HarfRust.

A few roadblocks are the following:

1. Getting the GlyphExtents data: One of the functions require Glyph Extents. This does not seem to be exposed. It *might* be just as simple as adding it to a `pub use` to `lib.rs`.

2. Getting the internals of `UnicodeBuffer`. This needs to be reset this from raw data coming from the module. IF there is a better way to do this I am all ears.

```
store_data.buffer.len = buffer.length as usize;

store_data.buffer.info.clear();
store_data.buffer.info.extend_from_slice(bytemuck::cast_slice(
&mem_data[buffer.info as usize..buffer.info as usize + array_length],
));

store_data.buffer.pos.clear();
store_data.buffer.pos.extend_from_slice(bytemuck::cast_slice(
&mem_data[buffer.position as usize..buffer.position as usize + array_length],
));
```

3. Last and least: `hb_font_t`, exported as `Shaper`, seems to have lost the inquiries about specific glyphs and tables. I am not quite sure how to access these. (I have not thought about this too much, and it might be as simple as just passing the `FontRef` from `read_fonts` separately.

Am I missing something obvious in regards to these points or are they something the crate can export?