Handle doc comments and empty variants in the Command! macro

This patch introduces two changes to the Command! macro:
- We allow variants without fields so that we no longer have to define
  empty *Args structs just for the Command! macro.
- We allow doc comments so that we can document commands without a
  separate *Args struct.

src/arg_util.rs

@@ -24,12 +24,21 @@ macro_rules! count {
  }
 }
 
+/// Translate an optional source into an optional destination.
+macro_rules! tr {
+ ($dst:tt, $src:tt) => {
+ $dst
+ };
+ ($dst:tt) => {};
+}
+
 macro_rules! Command {
- ( $name:ident, [ $( $var:ident($inner:ident) => $exec:expr, ) *] ) => {
+ ( $name:ident, [ $( $(#[$doc:meta])* $var:ident$(($inner:ty))? => $exec:expr, ) *] ) => {
  #[derive(Debug, PartialEq, structopt::StructOpt)]
  pub enum $name {
  $(
- $var($inner),
+ $(#[$doc])*
+ $var$(($inner))?,
  )*
  }
 
@@ -41,7 +50,7 @@ macro_rules! Command {
  ) -> crate::Result<()> {
  match self {
  $(
- $name::$var(args) => $exec(ctx, args),
+ $name::$var$((tr!(args, $inner)))? => $exec(ctx $(,tr!(args, $inner))?),
  )*
  }
  }