Consider changing MemoryDB::new to take no arguments

[expenses]

See paritytech/substrate#1427. MemoryDB is different from a lot of Rust storage types in that the new function takes an argument. I think it would make more sense for MemoryDB::new to construct an empty DB, alike, MemoryDB::default. A new from_data function could be added that replaces the old new function:

pub fn from_data(data: &'a [u8]) -> Self {
    Self::from_null_node(data, data.into())
}