Make build, check and docs target directory an (optional) argument

Instead of being an option. Feels more natural that way.

crates/aiken/src/cmd/build.rs

@@ -4,7 +4,6 @@ use std::path::PathBuf;
 /// Build an Aiken project
 pub struct Args {
     /// Path to project
-    #[clap(short, long)]
     directory: Option<PathBuf>,
 
     /// Also dump textual uplc

crates/aiken/src/cmd/check.rs

@@ -4,7 +4,6 @@ use std::path::PathBuf;
 /// Type-check an Aiken project
 pub struct Args {
     /// Path to project
-    #[clap(short, long)]
     directory: Option<PathBuf>,
 
     /// Skip tests; run only the type-checker

crates/aiken/src/cmd/docs.rs

@@ -4,7 +4,6 @@ use std::path::PathBuf;
 /// Build an Aiken project
 pub struct Args {
     /// Path to project
-    #[clap(short, long)]
     directory: Option<PathBuf>,
 
     /// Output directory for the documentation