Switch RcStr from std::sync::Arc to triomphe::Arc (#8715)
# Description
This removes the weakref from each `Arc`, which we weren't using, saving
64 bits per unique `RcStr`.
https://docs.rs/triomphe/latest/triomphe/struct.Arc.html
The difference in memory is too small to easily measure with heaptrack,
but it should be a small win regardless, and it's a drop-in replacement
that doesn't make the code any more complicated.
# Testing Instructions
Built using
```
cargo build --release -p next-build-test
```
Ran heaptrack on a single page (`/sink`) in `shadcn/ui` with:
```
cd ~/ui/apps/www
RUST_LOG=next_build_test=info heaptrack ~/nextpack/target/release/next-build-test run sequential 1 1 '/sink'
```
And analyzed the results with
```
heaptrack --analyze /home/bgw.linux/ui/apps/www/heaptrack.next-build-test.3066837.zst | tail -100
```
## Heaptrack Before PR
```
total runtime: 135.14s.
calls to allocation functions: 49700807 (367770/s)
temporary memory allocations: 4126130 (30532/s)
peak heap memory consumption: 1.18G
peak RSS (including heaptrack overhead): 2.76G
total memory leaked: 11.93M
suppressed leaks: 7.24K
```
```
total runtime: 138.50s.
calls to allocation functions: 49771465 (359355/s)
temporary memory allocations: 4410274 (31842/s)
peak heap memory consumption: 1.18G
peak RSS (including heaptrack overhead): 2.75G
total memory leaked: 12.09M
suppressed leaks: 7.24K
```
## Heaptrack After PR
```
total runtime: 145.38s.
calls to allocation functions: 49714711 (341966/s)
temporary memory allocations: 4164393 (28645/s)
peak heap memory consumption: 1.18G
peak RSS (including heaptrack overhead): 2.75G
total memory leaked: 11.67M
suppressed leaks: 7.24K
```
```
total runtime: 128.03s.
calls to allocation functions: 49739679 (388515/s)
temporary memory allocations: 4111893 (32117/s)
peak heap memory consumption: 1.18G
peak RSS (including heaptrack overhead): 2.76G
total memory leaked: 11.88M
suppressed leaks: 7.24K
```
