The function crypto_generichash_statebytes exists to dynamically determine the
size of a crypto_generichash_state struct. This is useful when using libsodium
from a language which can't use sizeof on C types. However, no equivalent
existed for the crypto_generichash_blake2b_state struct for users who want to
explicitly use the blake2b algorithm.
The function crypt_generichash_blake2b_statebytes is added to fill this gap.
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