Minor new features and tweaks

In this release:

• Added operator-> support to result<T>;
• Added a new example to demonstrate using LEAF with outcome::result<T> instead of leaf::result<T>;
• LEAF_AUTO uses .value() instead of operator* to access the value of a result<T> type;
• Fixed a bug in the single header generation script, now headers are included only once;
• Documentation fixes and refinements.