log: lots of optimizations to terminal output

- implement custom time writer,
- better padding with spaces
- better use of spaces
- log: more efficient handling of slog.Value
- log: utilize scratch-buffer in terminal handler
- log: use scratch-space for value formatting

Author: holiman

internal/testlog/testlog.go

@@ -183,8 +183,7 @@ func (h *bufHandler) terminalFormat(r slog.Record) string {
 	}
 
 	for _, attr := range attrs {
-		rawVal := attr.Value.Any()
-		fmt.Fprintf(buf, " %s=%s", attr.Key, log.FormatLogfmtValue(rawVal, true))
+		fmt.Fprintf(buf, " %s=%s", attr.Key, string(log.FormatSlogValue(attr.Value, true, nil)))
 	}
 	buf.WriteByte('\n')
 	return buf.String()