Commits on Feb 24, 2020

1. uses clap instead of custom cli arg parsing …

   This commit uses [clap](https://github.com/clap-rs/clap) to handle
   command line arugments instead of a custom rolled solution. The benefit
   that `clap` handles all validation as well as providing many "quality of
   life" or UX improvements such as hidden alias, suggestions on typo,
   auto-generated help, etc.
   
   A previous branch of this project used
   [structopt](https://github.com/TeXitoi/structopt) (which internally uses
   `clap`). `structopt` makes heavy use of Rust's Custom Derive feature to
   make the code less verbose and more ergonomic. `clap` is nearing it's v3
   release, which pulls `structopt` into the mainline code base. This
   commit does not use the v3 `structopt`-like feature-set since that
   release is still in alpha. This meanas the code is more verbose (yet
   just as performant), and could be changed to the `structopt`-like
   features once `clap` v3 is officially released.
   
   This commit also does not utilize some of `clap`'s other features like the
   ability to generate [Shell Completion scripts](https://docs.rs/clap/2.33.0/clap/struct.App.html#method.gen_completions)
   which could be added later if desired.
   
   This commit is *nearly* a 1:1 translation from the original home rolled,
   to using `clap`. I say "nearly" because `clap` adds many features on top
   of what the home rolled solution provided "for free" as in with no
   additional configuration. Those features include:
   
   * Argument validation: `--routes`, `--dns-servers`, and `--port` all now
   have built in validation. If someone enters an IP that isn't valid, or a
   port that isn't valid an error is returned with a friendly message prior
   to advancing into `gnirehtet` code. `--routes` also checks the CIDR for
   validity as well.
   * Auto generated help message
   * Suggestions on typos (this feature could be turned off to reduce the
   binary size, see below)
   * [Hidden Command aliases](https://docs.rs/clap/2.33.0/clap/struct.App.html#method.alias); The following hidden aliases have been added for
   commands:
     * `rt` which maps to `run`: I noticed `gnirehtet rt` used to be used for `gnirehtet run`
   * Long options for arguments (i.e. `--port` as well as `-p`)
   * [Hidden Argument aliases](https://docs.rs/clap/2.33.0/clap/struct.Arg.html#method.alias); the following hidden aliases have been added
   for arguments (which provides help for common typos):
     * `--dns-server` -> `--dns` -> `--dns-servers`: All of these map to the same thing
     * `--route` -> `--routes`
   * Long and Short help messages: `clap` allows one to specify a short
   message for arguments and commands when `-h` is used, and display a
   longer more comprehensive message when `--help` is used.
   * Multiple ways to get help: `clap` provides `-h`|`--help` for the base
   `gnirehtet` command, as well as all it's subcommands, i.e. `gnirehtet
   run --help`. There is also a `help` subcommand `gnirehtet help` which
   can be used alone or to display other commands, i.e. `gnirehtet help
   run`. This helps because people assume various forms of help, and should
   be able to find the help messages no matter how they originally try it.
   
   Binary Size:
   
   I see that binary size is a concern. This is the size after this commit:
   
   ```
   ❯ ls -l target/release/gnirehtet
   .rwxrwxr-x 2.2M kevin 23 Feb 13:22 target/release/gnirehtet
   ❯ strip -g target/release/gnirehtet
   ❯ ls -l target/release/gnirehtet
   .rwxrwxr-x 1.1M kevin 23 Feb 13:22 target/release/gnirehtet
   ❯ strip -s target/release/gnirehtet
   ❯ ls -l target/release/gnirehtet
   .rwxrwxr-x 973k kevin 23 Feb 13:22 target/release/gnirehtet
   ```
   
   So it does increase the binary size, but not dramatically. By further
   turning off [`clap`'s default cargo
   features](https://github.com/clap-rs/clap/#optional-dependencies--features),
   the size may be able to be reduced even more. However, I also am a firm believer that these slight
   increases are well worth the features they provide.
   
   Relates to Genymobile#243
   
   wip: clap

   

   kbknapp committed Feb 24, 2020
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