estd::intrusive_forward_list babysitting

[malachib]

There seems to be a confusion between estd::intrusive_forward_list and estd::internal::list::instrusive_forward

IIRC estd::intrusive_forward_list really IS, and estd::internal::list::instrusive_forward was underway but never fully finished to replace it. At a minimum put some commentary in there. May be an overlap with #11

Make public value_type and friends for estd::internal::list::instrusive_forward

In context to embr::audio unit processing:

for( : range) syntax has some issues (iterator returns value rather than reference) OR we may need more permissive/friendlier const handling - not yet determined

Add https://en.cppreference.com/w/cpp/container/forward_list/remove.html and also a custom remove_first and remove_first_if