New BiLong2ObjectMap implementation

[CarlosVara]

This commit introduces a new BiLong2ObjectMap collection, along with a BiLong2NullableObjectMap companion implementation. These two new maps are derived from the existing BiInt2ObjectMap equivalents that are already present in the collections package.

Implementation wise, the BiLong2ObjectMap keeps its keys in a single long[] array, with the 2 key parts stored in adjacent locations. This choice limits the maximum map capacity, but should provide better cache locality for key access.