Optimize convertPrintable().
I actually only came to silence sprintf() deprecation warnings, but
looking at
```
BM_log_convertPrintable_ascii 64.8 ns 64.7 ns 10837850
BM_log_convertPrintable_non_printable 247 ns 247 ns 2852322
BM_log_convertPrintable_non_ascii 6809 ns 6780 ns 100341
```
I thought we could do a lot better.
This patch replaces the sprintf() calls with manual two-digit hex
escaping (since we don't have a buffer size).
Oddly that made the "non-printable" case faster than the [printable]
"ASCII" case, so I added a __builtin_expect() to stop clang reordering
the different cases based on how complex the conditions are.
So now all three cases are faster than before:
```
BM_log_convertPrintable_ascii 56.5 ns 56.5 ns 12547408
BM_log_convertPrintable_non_printable 60.3 ns 60.3 ns 11531228
BM_log_convertPrintable_non_ascii 1542 ns 1542 ns 472010
```
Change-Id: I69e1ca3c8d0dbc831e533a166414f29be1b8dd08
1 file changed
