Unified line coverage formula for measurement and comparison.

[DonggeLiu]

E.g., the percentage line coverage of a new fuzz target f1 should be the #lines it covered over the #lines it linked during compilation:
$$\frac{Cov(f1)}{Linked(f1)}$$

The percentage line coverage improvement of f1 over an existing fuzz target f0 should be the #lines covered by f1 minus the #lines covered by f2, then divided by the union #lines linked by both.
$$\frac{Cov(f1)-Cov(f0)}{Linked(f1 \cup f0)}$$