Add support for crypto_kdf_hkdf_sha256* functions.

This release adds support to the crypto_kdf_hkdf_sha256* functions added in libsodium 1.0.19.

This release also fixes a bug in some keygen() functions which return the size (32) instead of 0|1 for success/fail, these were wrongly checked before. thanks to @stsch9 for their contribs!