Problem:
- The writer has to be supplied in the log config; there are use cases for
  changing the writer in the environment. For instance this would provide an
  alternative, more flexible way to do secure logging.
- The hardcoded dichotomy between `log_by_args` and `log_by_buf` is annoying and
  a relic of the original implementation.

Solution:
- Allow the log writer to be passed in the log environment.
- Instead of `log_by_args` and `log_by_buf`, a writer should provide a function
  that takes the span to write. Now the callee can choose whether to make that a
  function template and potentially unroll the write loop, or just a function
  taking a dynamic span with a runtime loop.