Replace argparse with structopt

This patch changes the argument handling code to use structopt instead
of argparse using the data structures we introduced in the last patch.
As part of that transition we replace the old Error::ArgparseError
variant with ClapError that stores a structopt::clap::Error.

Because of that replacement, the format of the help messages changed,
breaking some of the tests. Hence, this change adapts them accordingly.
Also clap currently prints the version output to stdout, so we ignore
the version_option test case for now.

CHANGELOG.md

@@ -2,6 +2,7 @@ Unreleased
 ----------
 - Reworked argument handling:
  - Added `structopt` dependency in version `0.3.7`
+ - Replaced `argparse` with `structopt`
 - Removed vendored dependencies and moved source code into repository
  root
 

src/arg_util.rs

@@ -67,11 +67,10 @@ macro_rules! Command {
  fn execute(
  self,
  ctx: &mut crate::args::ExecCtx<'_>,
- args: ::std::vec::Vec<::std::string::String>,
  ) -> crate::Result<()> {
  match self {
  $(
- $name::$var(_) => $exec(ctx, args),
+ $name::$var(args) => $exec(ctx, args),
  )*
  }
  }