Cache improvement

[vortigont]

• cached node link updates on add/remove also
• ListNode has a constructor now, simplifies code, ensures *next is a nullptr if unlinked
• some code sanity checks and cleanup
• add head()/tail()/exist() helpers to make user code simple on Node access/checks
• unit tests updated for new helpers

[ivanseidel]

Really interesting. Its missing a few docs for people to know how to use the new methods. Also, have you tested on arduino platform? is it compliant?

[vortigont]

for sure it works pretty fine with Arduino. Overloaded constructors mostly used internally to optimize node adding/creating, not much of a use for user actually, unless you want to create a list of lists :))
The other methods like head()/tail() are pretty obvious, but could be mentioned in examples also.