Add support for coverage counters.

Add support for coverage counters.

This commit implements a simple form of coverage instrumentation.  Points
in source code that are of interest must be explicitly annotated with
COVERAGE_INC.  The coverage counters may be logged at any time with
coverage_log().

This form of coverage instrumentation is intended to be so lightweight that
it can be enabled in production builds.  It is obviously not a substitute
for traditional coverage instrumentation with e.g. "gcov", but it is still
a useful debugging tool.