feat: add as type casting

tests/cast_int_to_float.test

@@ -0,0 +1,24 @@
+[code]
+fn cast_i32_to_f32(x: i32) -> f32 {
+    return x as f32;
+}
+
+fn cast_u64_to_f64(x: u64) -> f64 {
+    return x as f64;
+}
+
+[harness]
+#include <stdint.h>
+
+extern float cast_i32_to_f32(int32_t x);
+extern double cast_u64_to_f64(uint64_t x);
+
+int main() {
+    if (cast_i32_to_f32(42) != 42.0f) return 1;
+    if (cast_i32_to_f32(-10) != -10.0f) return 2;
+    if (cast_u64_to_f64(1337) != 1337.0) return 3;
+    return 0;
+}
+
+[expected_return_code]
+0