Shared trie

[maxitg]

Changes

• Use shared trie for a range evaluation.
• This uses more memory but saves a lot of time because the same evaluation paths ("highways") don't need to be evaluated multiple times.
• To accommodate this case, MergedWithAnotherInit was added as a ConclusionReason.

Comments

• There is currently no way to find out which other init this one was merged into. This potentially can be added, but it will require a change in the file format, so I did not add it now.
• A better approach might be to make this optimization invisible by automatically figuring out the values of all evaluation result components based on the information about the merge. I can work on this approach now, but I'm not sure I can finish it today (so it might have to wait until next week).

Examples

• The difference in performance can be seen with a disabled test:

TEST(PostTagSearcher, DISABLED_rangePerformance) {
  // With separate tries: 0.36 GB, 1133 seconds
  // With shared trie: 1.1 GB of RAM, 93 seconds, 12x speedup, 3x more memory use
  PostTagSearcher().evaluateRange(30, 0, 1000000, PostTagSearcher::EvaluationParameters());
}

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This change is