Make LibCrypto::Bio an opaque type like in the C header

The public headers don't disclose the data layout of `BIO`, so we should
not model it in the Crystal bindings either.

Doing that and having it wrong leads to issues such as crystal-lang#12647

src/openssl/lib_crypto.cr

@@ -57,28 +57,7 @@ lib LibCrypto
   type X509_STORE = Void*
   type X509_STORE_CTX = Void*
 
-  struct Bio
-    method : Void*
-    callback : BIO_callback_fn
-    {% if compare_versions(LIBRESSL_VERSION, "3.5.0") >= 0 %}
-      callback_ex : BIO_callback_fn_ex
-    {% end %}
-    cb_arg : Char*
-    init : Int
-    shutdown : Int
-    flags : Int
-    retry_reason : Int
-    num : Int
-    ptr : Void*
-    next_bio : Void*
-    prev_bio : Void*
-    references : Int
-    num_read : ULong
-    num_write : ULong
-  end
-
-  alias BIO_callback_fn = (Bio*, Int, Char*, Int, Long, Long) -> Long
-  alias BIO_callback_fn_ex = (Bio*, Int, Char, SizeT, Int, Long, Int, SizeT*) -> Long
+  type Bio = Void
 
   PKCS5_SALT_LEN     =  8
   EVP_MAX_KEY_LENGTH = 32