Unify usage of Optional instances

[guillermocalvo]

While most of the codebase uses japicmp's custom Optional type, some other parts rely on Guava's Optional implementation. This duality is somewhat confusing and may make the code hard to maintain long-term.

Since the minimum supported Java version is 8, I propose to only use Java's Optional type for simplicity. This allows for further improvements in the business logic.

Important

This refactor would be a major change in terms of backward compatibility, because the Optional type is exposed in the API.

These are some of the differences between the Optional types currently being used:

• Optional.absent() becomes Optional.empty().
• Optional.fromNullable(T) becomes Optional.ofNullable(T).
• Optional.or(T) becomes Optional.orElse(T).
• Guava's Optional type is serializable; the other implementations aren't.