:sig
String hmac_sha256(String key, String data)

:params
key : secret key
data : message to authenticate
return value : the raw 32-byte HMAC-SHA256

:content
Computes an HMAC-SHA256 and returns it as raw bytes. Prefer `hmac_sha256_hex()` for printable signatures; use the raw form for binary protocols.

:example
print(hmac_sha256("key", "message").length(), " raw bytes, hex = ", hmac_sha256_hex("key", "message"), "\n");

:see
>sys
hmac_sha256_hex
sha256
crypto_equal
