Logging: change ?LOG macro to ?WF_LOG

[codebykat]

This addresses the issue raised in #136. With props to @bunnylushington as I worked from your proposed solution.

See Slack discussion here: https://erlanger.slack.com/archives/CFV94TLCA/p1629216894001400

I have my doubts about two specifics:

• I'm undecided about continuing to define LOG when the OTP logger has not been included -- since the parameters differ, it might introduce a more confusing incompatibility than removing it entirely, and encouraging any consumers to move to WF_LOG explicitly.
• I also somewhat dislike the check for whether the OTP logger is defined, which introduces a requirement that the OTP logger must be included before the Nitrogen library. I'm not sure if there's precedent for forcing a particular order of library inclusion.

I'm wondering if it might be better to avoid the ifndef entirely and force usage of the new logger -- would it make sense to import logger.hrl as a dependency of Nitrogen? (This may be a naive question, as I'm new to Nitrogen and to the Erlang ecosystem.)

n.b. This PR does not update wf:info, wf:error etc to use the new macro.

@choptastic please let me know what you think, and what further work might be needed. Thanks!

[choptastic]

This is great! Thank you so much for putting in the work!

Forcing the new logger is definitely a good thing, but I have a thing about retaining backwards compatibility as much as possible, so for now, we'll definitely keep the ifndef block and the ?LOG macro around for older OTP versions.

Thanks again!