This makes copying such types much cheaper and will allow us to
intern common struct types in the future too. This further
drops the sample cost for langtype.rs from ~6.6% down to 4.0%.
We are now also able to share/intern common struct types.
Before:
```
Time (mean ± σ): 1.073 s ± 0.021 s [User: 0.759 s, System: 0.215 s]
Range (min … max): 1.034 s … 1.105 s 10 runs
allocations: 3074261
```
After:
```
Time (mean ± σ): 1.034 s ± 0.026 s [User: 0.733 s, System: 0.201 s]
Range (min … max): 1.000 s … 1.078 s 10 runs
allocations: 2917476
```
This removes a lot of allocations and speeds up the compiler step
a bit. Sadly, this patch is very invasive as it touches a lot of
files. That said, each individual hunk is pretty trivial.
For a non-trivial real-world example, the impact is significant,
we get rid of ~29% of all allocations and improve the runtime by
about 4.8% (measured until the viewer loop would start).
Before:
```
Benchmark 1: ./target/release/slint-viewer ../slint-perf/app.slint
Time (mean ± σ): 664.2 ms ± 6.7 ms [User: 589.2 ms, System: 74.0 ms]
Range (min … max): 659.0 ms … 682.4 ms 10 runs
allocations: 4886888
temporary allocations: 857508
```
After:
```
Benchmark 1: ./target/release/slint-viewer ../slint-perf/app.slint
Time (mean ± σ): 639.5 ms ± 17.8 ms [User: 556.9 ms, System: 76.2 ms]
Range (min … max): 621.4 ms … 666.5 ms 10 runs
allocations: 3544318
temporary allocations: 495685
```
