Implement missing Python functions

[randombit]

Following functions are defined in the C interface but not used in the Python binding:

• [ ] botan_constant_time_compare
• [ ] botan_hex_encode
• [ ] botan_hex_decode
• [ ] botan_base64_encode
• [ ] botan_base64_decode
• [x] botan_hash_copy_state
• [x] botan_hash_block_size
• [ ] botan_cipher_output_length
• [ ] botan_pwdhash
• [ ] botan_pwdhash_timed
• [x] botan_block_cipher_init
• [x] botan_block_cipher_destroy
• [x] botan_block_cipher_clear
• [x] botan_block_cipher_set_key
• [x] botan_block_cipher_block_size
• [x] botan_block_cipher_encrypt_blocks
• [x] botan_block_cipher_decrypt_blocks
• [x] botan_block_cipher_name
• [x] botan_block_cipher_get_keyspec
• [ ] botan_mp_set_from_radix_str
• [x] botan_mp_to_bin
• [ ] botan_mp_from_bin
• [x] botan_mp_is_positive
• [x] botan_mp_is_zero
• [ ] botan_mp_mod_mul
• [x] botan_mp_rand_bits
• [x] botan_mp_rand_range
• [ ] botan_mp_gcd
• [ ] botan_privkey_ed25519_get_privkey
• [ ] botan_privkey_export_encrypted
• [ ] botan_privkey_export_encrypted_pbkdf_iter
• [x] botan_privkey_load_dh
• [x] botan_privkey_load_dsa
• [x] botan_privkey_load_ecdh
• [x] botan_privkey_load_ecdsa
• [ ] botan_privkey_load_ed25519
• [x] botan_privkey_load_elgamal
• [x] botan_privkey_load_rsa
• [ ] botan_privkey_load_rsa_pkcs1
• [x] botan_privkey_load_sm2
• [ ] botan_privkey_load_x25519
• [ ] botan_privkey_rsa_get_privkey
• [ ] botan_privkey_x25519_get_privkey
• [ ] botan_pubkey_ed25519_get_pubkey
• [x] botan_pubkey_load_dh
• [x] botan_pubkey_load_dsa
• [x] botan_pubkey_load_ecdh
• [x] botan_pubkey_load_ecdsa
• [ ] botan_pubkey_load_ed25519
• [x] botan_pubkey_load_elgamal
• [x] botan_pubkey_load_sm2
• [ ] botan_pubkey_load_x25519
• [ ] botan_pubkey_x25519_get_pubkey
• [x] botan_privkey_check_key
• [x] botan_privkey_export_encrypted_pbkdf_msec
• [x] botan_privkey_get_field
• [x] botan_pubkey_check_key
• [x] botan_pubkey_get_field
• [x] botan_pubkey_load_rsa
• [ ] botan_pubkey_sm2_compute_za
• [x] botan_pk_op_key_agreement_size
• [ ] botan_pkcs_hash_id
• [ ] botan_x509_cert_dup
• [ ] botan_x509_cert_not_before
• [ ] botan_x509_cert_not_after
• [ ] botan_x509_cert_get_issuer_dn
• [ ] botan_x509_cert_allowed_usage
• [ ] botan_x509_cert_hostname_match
• [ ] botan_x509_cert_verify
• [ ] botan_totp_init
• [ ] botan_totp_destroy
• [ ] botan_totp_generate
• [ ] botan_totp_check

Probably not needed (redundant with other APIs, or not useful in Python):

• botan_scrub_mem
• botan_mp_equal
• botan_mp_set_from_int
• botan_mp_is_odd
• botan_mp_is_even
• botan_mp_add_u32
• botan_mp_sub_u32
• botan_mp_swap
• botan_mp_to_uint32
• botan_privkey_create_rsa
• botan_privkey_create_ecdsa
• botan_privkey_create_ecdh
• botan_privkey_create_mceliece
• botan_privkey_create_dh
• botan_privkey_create_dsa
• botan_privkey_create_elgamal
• botan_privkey_rsa_get_p
• botan_privkey_rsa_get_q
• botan_privkey_rsa_get_d
• botan_privkey_rsa_get_n
• botan_privkey_rsa_get_e
• botan_pubkey_rsa_get_e
• botan_pubkey_rsa_get_n
• botan_privkey_dsa_get_x
• botan_pubkey_dsa_get_p
• botan_pubkey_dsa_get_q
• botan_pubkey_dsa_get_g
• botan_pubkey_dsa_get_y
• botan_pubkey_load_sm2_enc
• botan_privkey_load_sm2_enc