Fixes and updates

#708 Fix missing class key with friend.
#709 intrusive_list::iterator operator -> should not dereference pointer before return.
#710 Calling accept on etl::variant visits a copy instead of the original object.
#711 Added etl::is_forward_link, etl::is_bidirectional_link and etl::is_tree_link to intrusive links. Added Constructors, access and status member functions to intrusive links.
#712 Unable to use etl::optional with non-default-constructible POD objects.
#716 etl::variant (variant_varadic) default constructed state
#720 etl::optional rvalue value_or() broken
#722 Bitset accesses memory out of bounds
#725 variant_pool create method should use etl::forward
Modified etl::visitor to allow direct specification of the argument type.
Added etl::is_visitor trait.
Removed default link type for etl::intrusive_list and etl::intrusive_forward_list

Note: The updates to etl::visitor changes how arguments are passed. Previously arguments would always be passed by reference. The new behaviour is to pass the arguments exactly as the template parameter specifies.

The declaration below defines virtual visit functions that accept Square&, Circle& and const Triangle&
etl::visitor<Square&, Circle&, const Triangle&>