write spaces in batches
experiment with alternative MemWriters

test bench_log::bench_copy                            ... bench:        87 ns/iter (+/- 30) = 7264 MB/s
test bench_log::bench_decoder                         ... bench:     26596 ns/iter (+/- 1725) = 23 MB/s
test bench_log::bench_direct_mem_writer               ... bench:      2210 ns/iter (+/- 94) = 209 MB/s
test bench_log::bench_direct_my_mem_writer            ... bench:      1939 ns/iter (+/- 120) = 239 MB/s
test bench_log::bench_encoder                         ... bench:      6988 ns/iter (+/- 488) = 90 MB/s
test bench_log::bench_manual_mem_writer_escape        ... bench:      2809 ns/iter (+/- 332) = 203 MB/s
test bench_log::bench_manual_mem_writer_no_escape     ... bench:      1817 ns/iter (+/- 113) = 315 MB/s
test bench_log::bench_manual_my_mem_writer2_escape    ... bench:      2250 ns/iter (+/- 117) = 254 MB/s
test bench_log::bench_manual_my_mem_writer2_no_escape ... bench:      1493 ns/iter (+/- 39) = 383 MB/s
test bench_log::bench_manual_my_mem_writer_escape     ... bench:      2196 ns/iter (+/- 47) = 260 MB/s
test bench_log::bench_manual_my_mem_writer_no_escape  ... bench:      1402 ns/iter (+/- 85) = 408 MB/s
test bench_log::bench_serializer                      ... bench:      2357 ns/iter (+/- 176) = 204 MB/s
test bench_log::bench_serializer2                     ... bench:      2070 ns/iter (+/- 38) = 233 MB/s
update to rust HEAD
undo commenting out writing json control characters
more experimentation with MemWriter variations