[Performance] Implement ability to snapshot memory to improve load times

[gorhill]

Memory snapshots (aka selfies) are not implemented in uMatrix -- unlike with uBO.

Implement:

• Hosts files selfie
• Public Suffix List selfie

This should improve load times, and memory usage, as seen with uBO.

[gorhill]

Benchmark is launch time with all stock hosts files selected, 84,857 distinct hostnames (8,322 duplicates were rejected but still needed parsing).

Before, 718.2 ms:

After, 127.6 ms:

When loading from selfies, the peak memory at launch time is vastly reduced as per Chromium's Task manager: from slightly over 60 MB (garbaged-collected to ~32 MB) to 28 MB (garbage-collected to ~26 MB).