Macro-ify benches and unify days list to one file

Also update README with new options.

src/days.rs

@@ -0,0 +1,12 @@
+// Single source of truth for all implemented days
+// Add new days here and they'll automatically be available in both the runner and benchmarks
+
+#[macro_export]
+macro_rules! all_days {
+    ($macro_name:path) => {
+        $macro_name! {
+            1 => day01,
+            2 => day02,
+        }
+    };
+}

src/lib.rs

@@ -1,2 +1,3 @@
 pub mod day01;
 pub mod day02;
+pub mod days;

src/main.rs

@@ -1,7 +1,7 @@
+mod days;
 mod runner;
 
-pub mod day01;
-pub mod day02;
+use aoc::{day01, day02};
 
 use clap::Parser;
 use color_eyre::Result;
@@ -24,10 +24,7 @@ struct Args {
     part: Option<u8>,
 }
 
-runner::days! {
-    1 => day01,
-    2 => day02,
-}
+all_days!(runner::days);
 
 fn main() -> Result<()> {
     color_eyre::install()?;