Proof-of-concept using mmap instead of manually buffered streams

[chhtz]

Related to #33 @planthaber
This appears to work, but I haven't tested it extensively (and not really benchmarked either), and only on Ubuntu 24, with rock_display_bin (which may not be 100% stable as well)

The index-file could probably also be switched to an mmap.

I'm not sure what happens if

• multiple users access the same file
• files get really big

[chhtz]

MappedStream is maybe not the best name, since it does not "stream". If the index-file can use this as well, then FileStream could be dropped.

I copied and edited FileStream.{hpp,cpp} therefore some comments are wrong, of course.

[chhtz]

Further TODOs:

• mmap index file
• Check impact of (other) flags for mmap. MAP_POPULATE could improve read-ahead (https://man7.org/linux/man-pages/man2/mmap.2.html)
  • Alternatively, try to read-ahead with madvise (https://man7.org/linux/man-pages/man2/madvise.2.html)
• Do munmap after usage (i.e., in destructor)
• Theoretically, std::copy could be avoided, since the data is in virtual memory already.