Create an interface for the cd entry hash table

The current implementation of the hashtable uses less memory than
a std::map. As most of the zip files we encountered don't use the zip64
extension, we should keep the current implementation. And the interface
adds the flexibility for us to switch to std::map for zip64 format.

Bug: 150900468
Test: unit tests pass
Change-Id: Ifd008785c9ff416a27049f9e0c54d9eef985bd85
2 files changed