feat: add as type casting

tests/cast_float_to_float.test

@@ -0,0 +1,21 @@
+[code]
+fn promote_f32_to_f64(x: f32) -> f64 {
+    return x as f64;
+}
+
+fn demote_f64_to_f32(x: f64) -> f32 {
+    return x as f32;
+}
+
+[harness]
+extern double promote_f32_to_f64(float x);
+extern float demote_f64_to_f32(double x);
+
+int main() {
+    if (promote_f32_to_f64(3.5f) != 3.5) return 1;
+    if (demote_f64_to_f32(2.25) != 2.25f) return 2;
+    return 0;
+}
+
+[expected_return_code]
+0