sql: optimize trigram generator

[mgartner]

The latency difference between our show_trgm builtin and Postgres's suggestions that there may be opportunities to optimize our trigram generation code. On my machine, the query select show_trgm('hello world') from generate_series(1, 10000); takes ~73ms in CRDB and ~14ms in PG.

Jira issue: CRDB-18836

[jordanlewis]

Hm I wonder what this is about. Here's what the code does:

1. find word boundaries
2. copy each word into a new allocated buffer (maybe this is too wasteful?) if running with padding, which show_trgm would be
3. make slices for all the trigrams
4. sort the trigrams
5. distinct the trigrams

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