Allow constant associated empty root value.

[cheme]

This PR adds new trait HasherNullEmptyRoot that keep trace of an associated null value for an empty type.
A new trait is used because this only makes sense if we assumes that the codec use in the trie will encode node to [0] (this is a bit fishy but rather straightforward solution).
Another memory db is implemented that uses this trait to avoid having to calculate the empty trie root (this happens a lot in substrate).
The implementation is generated from a macro and I am not very happy with the result, but again did not find any good and simple way to implement that, suggestions are very welcome.
Not that in the macro I did ommit an indentation on purpose to keep the diff readable, it should be added before merging, also did not do crate versioning update.

[burdges]

Is this for domain separation in the hash?

[cheme]

I am not sure about what you mean by "domain separation in the hash", but I it is probably not, it is just to avoid this kind of code https://github.com/paritytech/substrate/blob/faf608eeeda0031629f7608321c7ab41fac6024f/primitives/trie/src/lib.rs#L217 in substrate.
Edit: This is currently use to initialize the memorydb for child trie, so this allows to skip the calculation of the empty root which happen on new child trie but also one first editing of all child trie content (the overlay that store delta is a memory db. Thinking twice the overlay could switch to a type that does not implement the null node optimisation, that would be another option.