refactor(turbo-rcstr): Move turbo_tasks::RcStr into it's own crate (#72858)
The `RcStr` type doesn't use anything in `turbo-tasks`, so there's no reason it can't be a separate crate that doesn't depend on `turbo-tasks`.
This will let it be used in wasm swc plugins, where turbo-tasks does not currently compile.
Imports were automatically rewritten with:
```
# Fix imports:
sg run --lang rust --pattern 'use $CRATE::{$$$PRE, RcStr};' --rewrite 'use $CRATE::{$$$PRE}; use turbo_rcstr::RcStr;' --update-all
sg run --lang rust --pattern 'use $CRATE::{RcStr, $$$POST};' --rewrite 'use $CRATE::{$$$POST}; use turbo_rcstr::RcStr;' --update-all
sg run --lang rust --pattern 'use $CRATE::{$$$PRE, RcStr, $$$POST};' --rewrite 'use $CRATE::{$$$PRE, $$$POST}; use turbo_rcstr::RcStr;' --update-all
sg run --lang rust --pattern 'use $CRATE::RcStr;' --rewrite 'use turbo_rcstr::RcStr;' --update-all
cargo fmt
# Fix strings in comments:
fastmod crate::RcStr turbo_rcstr::RcStr
fastmod turbo_tasks::RcStr turbo_rcstr::RcStr
```
I had to manually fix the `Cargo.toml` files based on the compilation errors.