coverage: Include the highest counter ID seen in .cov-map dumps

When making changes that have a large impact on coverage counter creation, this
makes it easier to see whether the number of physical counters has changed.

(The highest counter ID seen in coverage maps is not necessarily the same as
the number of physical counters actually used by the instrumented code, but
it's the best approximation we can get from looking only at the coverage maps,
and it should be reasonably accurate in most cases.)

tests/coverage/unicode.cov-map

@@ -22,6 +22,7 @@ Number of file 0 mappings: 9
     = ((c0 - c2) + c3)
 - Code(Expression(3, Add)) at (prev + 2, 5) to (start + 1, 2)
     = (c4 + ((c0 - c2) + c3))
+Highest counter ID seen: c4
 
 Function name: unicode::他 (unused)
 Raw bytes (9): 0x[01, 01, 00, 01, 00, 1e, 19, 00, 25]
@@ -30,6 +31,7 @@ Number of files: 1
 Number of expressions: 0
 Number of file 0 mappings: 1
 - Code(Zero) at (prev + 30, 25) to (start + 0, 37)
+Highest counter ID seen: (none)
 
 Function name: unicode::申し訳ございません
 Raw bytes (9): 0x[01, 01, 00, 01, 01, 18, 01, 02, 02]
@@ -38,4 +40,5 @@ Number of files: 1
 Number of expressions: 0
 Number of file 0 mappings: 1
 - Code(Counter(0)) at (prev + 24, 1) to (start + 2, 2)
+Highest counter ID seen: c0