Include source info when writing logs

Before:
2023-07-12T15:59:18.091023Z: ThreadId(1): DEBUG: 7624 glyphs identified
After:
[2023-07-12T15:58:38.552647Z ThreadId(1) ufo2fontir::source DEBUG] 7624 glyphs identified`

So this is slightly more verbose, but knowing where a message was logged
from is very helpful when debugging. If we want to shave more characters
we can use millis instead of micros in the timestamp, and assign custom
thread names in the threadpool.

fontc/src/main.rs

@@ -12,9 +12,9 @@ fn main() -> Result<(), Error> {
             let ts = buf.timestamp_micros();
             writeln!(
                 buf,
-                "{}: {:?}: {}: {}",
-                ts,
+                "[{ts} {:?} {} {}] {}",
                 std::thread::current().id(),
+                record.target(),
                 buf.default_level_style(record.level())
                     .value(record.level()),
                 record.args()