WIP: Remove lots of unsafe from Node

[leshow]

Not sure if this is of interest to you, but I was able to remove large amounts of unsafe from Node. I used your benchmarks, and it runs about the same speed on my machine as before.

There are a few caveats:

• it uses smallvec for the label, which means the label will stay on the stack until it reaches len 10, then afterwards will be moved to the heap. There are a bunch of different sizes that I could've picked here but 10 seemed good for my purposes.
• it passes cargo miri test which checks for undefined behaviour, previously patricia_tree would just crash.
• I never used the binary_format feature so that part isn't working at all yet and has been removed from my code, I imagine you may want to add that back before something like this merges

The parts that really worried me were the un-aligned allocation combined with ptr::write which the std lib says causes UB. the use of transmute_copy too, which the docs say is one of the most unsafe things you can do, and could be error prone in edge cases.

[leshow]

I need to commit other things to master, I will re-open later if there is interest