Add detection of packers based on entropy

Currently, packers can be detected only by YARA rules or heuristics implemented in cpdetect.
Deciding whether binary is packed can be improved by considering entropy of input binary.

Start by reading the existing YARA-based detection and the heuristics implemented in cpdetect. Determine how entropy from the input binary could complement those mechanisms, and consider the work complete when packed-binary detection demonstrably uses entropy alongside the current approaches.

Written by the indexing model from the issue text.