[xdebug-general] Use a flat array for line coverage. (take 2)

I revisited the line coverage data structure refactoring I did in the
summer. It still looks like a fairly easy 4% win in overall
performance when running PHPUnit's own test suite (with coverage).

https://github.com/derickr/xdebug/pull/15

Thanks for accepting my other patches! I didn't see any comments on
this particular bit, so I have no idea if it just lay forgotten or if
it's not stylistically correct or… Just wondering if there's anything
else I can do to help get this (or something like it) merged in (or
knowing that it's not interesting to the project as a whole).

Have a great day!

-- 
taa
/*eof*/