Add entity diagram and rewrite loader file for using the best compiler options on both Logtalk 2.x and Logtalk 3.x

The entity diagram was generated using the `diagrams` tool of Logtalk 3.x.

The rewritten loader file ensure the best performance for the compiled code on both Logtalk 2.x and Logtalk 3.x. This includes the using the meta-predicate and lambdas optimizing compiler.