Based on the number of unique shared strings as well as the available memory amount,
one strategy will be chosen over the other.
The algorithm is based on empirical data and super safe so it may need to be tuned.
In-memory implementation using SplFixedArray
Updated code and tests to support errors when reading XML nodes (useful when reading XML files used for attacks)
Removed LIBXML_NOENT option (which DOES substitute entities...)
Added test for Quadratic Blowup attack
Escaped line feed characters in shared strings before processing them.
This makes every string remain on one single line and therefore allow
fast retrieval
Replaced usages of "\n" by PHP_EOL
Added test for multiline strings
