Add str(int) and repr(int) fast path using i64 conversion (#7302)

* Add str(int) and repr(int) fast path using i64 conversion

- Skip __str__ method resolution for exact PyInt in PyObject::str()
- Use i64::to_string() for small integers, BigInt::to_string() for large ones
- ~36% improvement in str(int) benchmarks

* Extract PyInt::to_str_radix_10() to deduplicate i64 fast path logic

crates/vm/src/builtins/int.rs

@@ -267,6 +267,15 @@ impl PyInt {
         &self.value
     }
 
+    /// Fast decimal string conversion, using i64 path when possible.
+    #[inline]
+    pub fn to_str_radix_10(&self) -> String {
+        match self.value.to_i64() {
+            Some(i) => i.to_string(),
+            None => self.value.to_string(),
+        }
+    }
+
     // _PyLong_AsUnsignedLongMask
     pub fn as_u32_mask(&self) -> u32 {
         let v = self.as_bigint();
@@ -603,7 +612,7 @@ impl Comparable for PyInt {
 impl Representable for PyInt {
     #[inline]
     fn repr_str(zelf: &Py<Self>, _vm: &VirtualMachine) -> PyResult<String> {
-        Ok(zelf.value.to_string())
+        Ok(zelf.to_str_radix_10())
     }
 }
 

crates/vm/src/protocol/object.rs

@@ -375,6 +375,13 @@ impl PyObject {
             Ok(s) => return Ok(s.into_pyref()),
             Err(obj) => obj,
         };
+        // Fast path for exact int: skip __str__ method resolution
+        let obj = match obj.downcast_exact::<PyInt>(vm) {
+            Ok(int) => {
+                return Ok(vm.ctx.new_str(int.to_str_radix_10()));
+            }
+            Err(obj) => obj,
+        };
         // TODO: replace to obj.class().slots.str
         let str_method = match vm.get_special_method(&obj, identifier!(vm, __str__))? {
             Some(str_method) => str_method,