feat: speed up PrefixRecord loading

[wolfv]

I have tried out rayon to parallelize the loading of PrefixRecord files. The results look pretty good:

load_serially           time:   [10.369 ms 10.403 ms 10.440 ms]
load_as_prefix_record   time:   [697.90 ns 699.63 ns 701.53 ns]
load_as_package_record  time:   [698.94 ns 700.45 ns 702.11 ns]

Unfortunately loading the subset (PackageRecord) doesn't seem to be a big improvement.

[wolfv]

For very long PrefixRecord's it does seem to make a difference (20k files):

load_long_prefix_record time:   [1.8900 ms 1.8961 ms 1.9029 ms]
load_long_package_record time:  [380.69 µs 382.43 µs 384.41 µs]