Change internal representation of commands into an `enum`

[dspinellis]

Rather than storing inside Command the command's character as code, represent each command as an enum.

So the existing:

pub struct Command {
    pub code: char,                         // Command code
    pub addr1: Option<Address>,             // Start address

will become

// Sed commands derived from single-character command codes
pub enum Command {
    Append,  // a
    Branch,  // b
    Change,  // c
    // …
}

pub struct CommandDetails {
    pub command: Command,                   // Command to perform
    pub addr1: Option<Address>,             // Start address