[noise] Minor KDF concern

[Stephen Touset]

While writing a basic implementation of Noise Boxes, I realized that the
KDF doesn't mix the output length into the HMAC inputs. If you use the KDF
to produce two keys of different lengths with the same secret, extra_data,
and info parameters, the shorter key will be a prefix of the longer one.

Does it make sense to append the key length to the message that is HMACed?
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