perf: serialize a directory entry to and from one 128 byte buffer

[francisdb]

A directory entry was written to the file field by field, in about 45 small writes (one per UTF-16 unit of the name, then one per field), and read the same way. On an unbuffered File each of those is a syscall. Every stream creation writes at least two entries, and the balanced insert from #83 touches a few more, so this is what the create-heavy cases on disk spend their time on; opening a file reads every entry the same way.

Both directions now go through a 128 byte buffer in memory and touch the file once. Measured against master with #83 merged:

case (criterion)	master	after
write to disk, 10000 x 0 B	510 ms	24 ms
write to disk, 1000 x 64 B	62 ms	3.4 ms
write to disk, 100 x 4 KiB-1 (mini stream)	50 ms	6.4 ms
write to disk, 100 x 4 KiB	5.6 ms	0.5 ms
write to memory, 10000 x 0 B	20 ms	8.2 ms
write to memory, 100 x 4 KiB	220 µs	94 µs

Reading from disk, with the read_streams_disk group from #94 (open the file, then read every stream):

case (criterion)	master	after
read from disk, 10000 x 0 B	48 ms	5.9 ms
read from disk, 1000 x 64 B	6.1 ms	1.1 ms
read from disk, 100 x 4 KiB-1 (mini stream)	3.2 ms	2.5 ms
read from disk, 100 x 4 KiB	841 µs	208 µs

The large-stream cases are unchanged either way, as expected. For reference, 0.14.0 sat at 625 ms, 40 ms, 67 ms and 2.9 ms on the four disk write cases, so the two that #83 made slower on disk end up well ahead of the release.