Rewrite argument parsing to use Clap's derives

This is something that has been on the radar for a while, but the recent
deprecation of everything from Clap 3.1 in Clap 3.2 has pushed our hand
a bit to get it done now with the new best practices from Clap 3.2.

Cargo.toml

@@ -64,11 +64,7 @@ dump_lookahead_data = ["byteorder", "image"]
 arg_enum_proc_macro = "0.3"
 bitstream-io = "1"
 cfg-if = "1.0"
-clap = { version = "3.1", optional = true, default-features = false, features = [
-    "color",
-    "std",
-    "wrap_help",
-] }
+clap = { version = "3.2.6", optional = true, default-features = false, features = ["color", "std", "wrap_help", "derive"] }
 clap_complete = { version = "3", optional = true }
 libc = "0.2"
 y4m = { version = "0.7", optional = true }

src/api/config/encoder.rs

@@ -118,7 +118,7 @@ pub struct EncoderConfig {
 /// [`with_speed_preset()`]: struct.EncoderConfig.html#method.with_speed_preset
 impl Default for EncoderConfig {
   fn default() -> Self {
-    const DEFAULT_SPEED: usize = 6;
+    const DEFAULT_SPEED: u8 = 6;
     Self::with_speed_preset(DEFAULT_SPEED)
   }
 }
@@ -130,7 +130,7 @@ impl EncoderConfig {
   /// than 10, it will result in the same settings as 10.
   ///
   /// [`from_preset()`]: struct.SpeedSettings.html#method.from_preset
-  pub fn with_speed_preset(speed: usize) -> Self {
+  pub fn with_speed_preset(speed: u8) -> Self {
     EncoderConfig {
       width: 640,
       height: 480,

src/api/config/speedsettings.rs

@@ -112,7 +112,7 @@ impl Default for SpeedSettings {
 
 impl SpeedSettings {
   /// Set the speed setting according to a numeric speed preset.
-  pub fn from_preset(speed: usize) -> Self {
+  pub fn from_preset(speed: u8) -> Self {
     // The default settings are equivalent to speed 0
     let mut settings = SpeedSettings::default();