Reduce memory usage for Coverage Data

[maks-rafalko]

This PR:

• Reduces memory usage when Coverage Data is being collected
• Covered by tests

Partially fixes issues from #705. At least I now able to run Infection for Psalm locally with memory_limit=12G :D

[sanmai]

PHP is very liberal with copying a whole array if you happen to change a single value, or if you wish to pass it around from function to function.

One way is to use references, but that's very troublesome and requires a great care. Another way is to wrap a big array with an object, and access this object's property instead.

[maks-rafalko]

Another way is to wrap a big array with an object, and access this object's property instead.

done in #710

Thank you @theofidry & @sanmai