stage2 大量优化代码使其自举编译时间大幅缩短

stage2/ir_builder.spl

@@ -14,8 +14,8 @@ type Ir = @import("./ir.spl");
 type .{ .IrKind, .ConstLit, .IrEntry, .IrFunc, .IrBody } = @import("./ir.spl");
 
 fn list_at(v: Vec, i: usize): usize {
-    match Vec.get(&v, i) { .Ok(p) => { ret (p as *usize).*; }, .Err(e) => { ret 0; } }
-    ret 0;
+    if i >= v.len { ret 0; }
+    ret ((v.data as *usize) + i).*;
 }
 
 // ---- builder ----