Decouple MemoryDB from RLP

[dvdplm]

Currently the memorydb crate has a hard dependency on rlp.

Make it generic over the encoding scheme using the NodeCodec trait from #8739.

[debris]

this is not trivial, because of how we treat null_node and null_node_rlp. it is exactly the same issue that prevents us from decoupling NodeCodec from rlp

#9043 (comment)

[dvdplm]

Good point.

[cheme]

Something along the lines of (using associated const)

#[derive(Clone, PartialEq)]
pub struct MemoryDB<M: MemoryDBConf> {
	data: FastMap<M::H, (DBValue, i32)>,
	hashed_null_node: <M::H as KeyHasher>::Out,
}

pub trait MemoryDBConf {
  const NULL_VALUE: &'static [u8];
  type H: KeyHasher;
}

could do it, but I feel that the added complexity (new trait with impact everywher) may not be worth it.

[cheme]

This issue is fixed now (memorydb contains a null node hashed instance and there is no more deps to rlp). Note that memory db is no longer in parity-ethereum but shared with substrate.