Dependency injection in Xvc

Almost all Xvc functions receive a channel in the form

fn cmd_api(output_snd: Sender<XvcOutputLine>, ...)

Functions use this to send messages to user.

XvcOutputLine is an enum in the form:

pub enum XvcOutputLine {
    /// The output that we should be reporting
    Output(String),
    /// For informational messages the user doesn't usually follow
    Info(String),
    /// Warnings that are against some usual workflows
    Warn(String),
    /// Errors that interrupts a workflow but may be recoverable
    Error(String),
    /// Panics that interrupts the workflow and ends the program
    /// Note that this doesn't call panic! automatically
    Panic(String),
    /// Progress bar ticks.
    /// Self::Info is also used for Tick(1)
    Tick(usize),
}

When a commands wants to send a text to the outside world or wants to increment a counter, it uses this enum.

Currently, all these outputs are written to the console. In the future, when other output channels (like web etc.) are created, these functions will return their output there.