arch refactor + backend ir start

src/backend/link.rs

@@ -0,0 +1,12 @@
+use crate::backend::{Addr, elf};
+
+pub struct LinkedProgram {
+    pub code: Vec<u8>,
+    pub entry: Option<Addr>,
+}
+
+impl LinkedProgram {
+    pub fn to_elf(&self) -> Vec<u8> {
+        elf::create(&self.code, self.entry.expect("no start"))
+    }
+}