Add openssl_cipher_key_length function

This function works in exactly the same way as openssl_cipher_iv_length
but for a key length. This is especially useful to make sure that the
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Add openssl_cipher_key_length function

This function works in exactly the same way as openssl_cipher_iv_length
but for a key length. This is especially useful to make sure that the
right key length is provided to openssl_encrypt and openssl_decrypt.

In addtion the change also updates implementation of
openssl_cipher_iv_length and adds a test for it.

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