Add accessors and writers for EVP_CIPHER_CTX

New functions:

- EVP_CIPHER_CTX_encrypting()
- EVP_CIPHER_CTX_iv()
- EVP_CIPHER_CTX_iv_noconst()
- EVP_CIPHER_CTX_original_iv()
- EVP_CIPHER_CTX_buf_noconst()
- EVP_CIPHER_CTX_num()
- EVP_CIPHER_CTX_set_num()
- EVP_CIPHER_CTX_cipher_data()
- EVP_CIPHER_CTX_new_cipher_data()

Note that the accessors / writers for iv, buf and num may go away, as
those rather belong in the implementation's own structure (cipher_data)
when the implementation would affect them (that would be the case when
they are flagged EVP_CIPH_CUSTOM_IV or EVP_CIPH_FLAG_CUSTOM_CIPHER).

Reviewed-by: Rich Salz <rsalz@openssl.org>