HMAC supports arbitrary key sizes, and there are no practical reasons
to use more than 256 bit keys.
It still makes sense to match the security level, though, especially
since a distinction between the block size and the key size can be
confusing.
Using HMAC.key_size instead of HMAC.mac_size caused our TLS
implementation to compute wrong shared secrets when SHA-384 was
used. So, fix it directly in `crypto.hmac` in order to prevent
other misuses.
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