perf(pg-protocol): decode short ascii values without buffer.toString()

[nigrosimone]

Every field of every row goes through BufferReader.string(), which calls buffer.toString('utf-8'). That call has a fixed cost in the C++ boundary that is bigger than the decoding itself when the value is short, and most values in a result set are short: ids, flags, small numbers, dates.

So for values up to 16 bytes that are all ASCII the string is now built with String.fromCharCode. Anything longer, or with a byte above 127, still goes to buffer.toString().

Parser only, 500 rows per message, median of 5 runs on node 26 (ms, lower is better):

5 short columns (id, fk, bool, int, 'ok'):   25.1 ->  8.3
5 columns with dates and codes:              28.4 -> 10.4
8 mixed columns (names, emails, timestamp):  39.1 -> 24.2
4 long text columns (37-80 chars):           22.2 -> 22.8
3 short non ascii columns:                   27.0 -> 27.6

End to end on localhost, SELECT of 5000 rows, 5 short columns: 9.1ms -> 6.7ms per query.

The test in inbound-parser.test.ts parses a data row with a value of every length from 0 to 19 plus accented, chinese and emoji values, and fails if the ascii check is removed.