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scylladb/utils
Avi Kivity 15fa1c1c7e Merge 'sstables/trie: translate all key cells in one go, not lazily' from Michał Chojnowski
Applying lazy evaluation to the BTI encoding of clustering keys
was probably a bad default.
The possible benefits are dubious (because it's quite likely that the laziness
won't allow us to avoid that much work), but the overhead needed to
implement the laziness is large and immediate.

In this patch we get rid of the laziness.
We rewrite lazy_comparable_bytes_from_clustering_position
and lazy_comparable_bytes_from_ring_position
so that they performs the key translation eagerly,
all components to a single bytes_ostream in one synchronous call.

perf_bti_key_translation (microbenchmark added in this series, 1 iteration is 100 translations of a clustering key with 8 cells of int32_type):
```
Before:
test                            iterations      median         mad         min         max      allocs       tasks        inst      cycles
lcb_mismatch_test.lcb_mismatch        9233   109.930us     0.000ns   109.930us   109.930us    4356.000       0.000   2615394.3    614709.6

After:
test                            iterations      median         mad         min         max      allocs       tasks        inst      cycles
lcb_mismatch_test.lcb_mismatch       50952    19.487us     0.000ns    19.487us    19.487us     198.000       0.000    603120.1    109042.9
```

Enhancement, backport not required.

Closes scylladb/scylladb#26302

* github.com:scylladb/scylladb:
  sstables/trie: BTI-translate the entire partition key at once
  sstables/trie: avoid an unnecessary allocation of std::generator in last_block_offset()
  sstables/trie: perform the BTI-encoding of position_in_partition eagerly
  types/comparable_bytes: add comparable_bytes_from_compound
  test/perf: add perf_bti_key_translation
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