Higher precision timestamps in maktaba#log#

[dbarnett]

We should be able to add sub-second precision by using reltime() instead of localtime() in vim builds with the reltime feature eanbled.

If that turns out to add significant overhead, we could guard it with a setting or have individual log messages opt in to precise timing.

[dbarnett]

This issue makes me realize that exposing the intricacies of the data types for log entries to callers outside maktaba makes it trickier to update the format. In this case, I wanted to change the timestamp type from a number to a float, but callers will see an error if they don't update to stop trying to pass that as-is to strftime().

I'm going to implement the log entry formatting inside maktaba first so that handlers can just pass opaque entries back to maktaba to manipulate them and don't need to depend on the data types. We talked about including more log handling logic inside maktaba as part of #141, anyway.