ditch structopt in favor of clap 3.0 beta1

It's bascially the same, but should make getting size improvements
way easier.

Author: Byron

Diff for: Cargo.lock

@@ -35,7 +35,7 @@ light = ["fast", "lean-cli", "git-features/interruptible"]
 small = ["lean-cli"]
 
 fast = ["git-features/parallel", "git-features/fast-sha1"]
-pretty-cli = ["structopt",
+pretty-cli = ["clap",
     "git-features/progress-prodash",
     "git-features/interruptible",
     "gitoxide-core/serde1",
@@ -61,7 +61,7 @@ anyhow = "1.0.31"
 gitoxide-core = { version = "0.1.0", path = "gitoxide-core" }
 git-features = { version = "^0.2.0", path = "git-features" }
 
-structopt = { version = "0.3.14", optional = true }
+clap = { version = "3.0.0-beta.1", optional = true }
 argh = { version = "0.1.3", optional = true, default-features = false }
 prodash = { version = "7.0.2", optional = true, default-features = false }
 atty = { version = "0.2.14", optional = true, default-features = false }

Diff for: Cargo.toml

@@ -245,7 +245,7 @@ The top-level command-line interface.
   * If disabled, the binary will be visibly smaller.
 * _(mutually exclusive)_
   * **pretty-cli**
-    * Use `clap` + `structopt` to build the prettiest, best documented and most user-friendly CLI at the expense of file size.
+    * Use `clap` 3.0 to build the prettiest, best documented and most user-friendly CLI at the expense of file size.
     * provides a terminal user interface for detailed and exhaustive progress.
     * provides a line renderer for log-like progress
   * **lean-cli**