core: #39: don't call logging framework when logging is disabled

It looks ugly as sin, but this nets about a 20% drop in user CPU time,
and close to 15% increase in throughput.

The average log call is around 10 opcodes, prefixing with '_v and' costs
an extra 2, but both are simple operations, and the remaining 10 are
skipped entirely when _v or _vv are False.