Use boxed types in A* data structures

[sebschmi]

Most runtime is spent on hashtable and heap operations. This is likely slowed by the fact that both contain Node objects, but those are larger than 100 bytes. If we would switch the data structures to use Box<Node> instead, that may speed things up.

[jeeeesper]

The estimation of used memory here needs to be adjusted as well after this change. Something like get_size will be handy here.

[sebschmi]

I see, thanks. That's actually a bug now.

[jeeeesper]

Solved with #93 -- See #92 for details.