perf(turbopack/rcstr): Use precomputed hash (#80074)
### What?
Use precomputed data for hashing instead of rehashing everytime.
### Why?
`hash` of `&str` is `O(n)`. Although it's fast, we don't need to pay the cost for memory access.
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
