Refactor Matter to be a Trait.

[m00sey]

As commented in #72

Matter would be better used abstractly.

Currently the matter implementation uses member variables for code/raw etc and mutates them internally.

We should add get_code() get_raw() to a Matter Trait and move the member variables up to Diger/Cigar/Verfer.

A Matter Trait could look something like:

pub trait Matter {
pub fn qb64b(&self) -> Result<Vec<u8>> //with default impl
pub fn qb2(&self) -> Result<Vec<u8>> //with default impl
pub fn code(&self) -> String
pub fn raw(&self) -> Vec<u8>

fn infil(&self) -> Result<String> //with default impl
fn binfil(&self) -> Result<Vec<u8>> //with default impl
fn exfil(&mut self, qb64: &str) -> Result<()> //with default impl
fn bexfil(&mut self, qb2: &[u8]) -> Result<()> //with default impl
}

[jasoncolburne]

+1