Reuse allocation of ks_asm

[tesuji]

Basically this:

-    pub fn asm(&self, insns: String, address: u64) -> Result<KeystoneOutput> {
+    pub fn asm(&self, insns: &CStr, address: u64) -> Result<KeystoneOutput> {
// and
 pub struct KeystoneOutput {
-    /// Size of the array storing the encoded instructions.
-    pub size: u32,
     /// Number of instructions that were successfully encoded.
-    pub stat_count: u32,
-    /// Array of encoded instructions.
-    pub bytes: Vec<u8>,
+    pub stat_count: size_t,
+    /// Size of the array storing the encoded instructions.
+    size: size_t,
+    /// A pointer of allocated encoded instructions.
+    ptr: *mut u8,
+}
+impl Drop for KeystoneOutput {
+    fn drop(&mut self) {
+        unsafe {
+            ffi::ks_free(self.ptr);
+        }
+    }
+}

From keystone-engine/keystone#589 but it stalled