Turbopack: store serialized sourcemaps instead (#75791)
Store sourcemaps JSON-serialized (as opposed to structs containing arrays of decoded mappings), which is more memory efficient (and the main operation we perform is concatenation anyway, lookups are only used for error message locations)
This will also deduplicate sourcemaps (because cloning a `Rope` is just a reference-counted pointer).
```
testing against 0293c96cf32
canary e5fc495e3d
19,1 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 526.06s user 87.43s system 853% cpu 1:11.91 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 510.69s user 82.93s system 835% cpu 1:11.06 total
sourcemap-rope 2dc238215a
16.94 GB
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 535.61s user 85.04s system 855% cpu 1:12.55 total
TURBO_ENGINE_READ_ONLY=1 NEXT_TURBOPACK_TRACING= TURBOPACK=1 TURBOPACK_BUILD= 531.07s user 82.25s system 861% cpu 1:11.22 total
```
