Adding new public key KEM API

[NIIBE Yutaka]

Hello,

Falko Strenzke <falko.strenzke at mtg.de> wrote:
> There is another point to consider for the design of a generic KEM API: 
> the use of the public in the key derivation, which makes it necessary to 
> pass the public key to the decapsulation function if one doesn't want to 
> run the computation of the public key from the private key in the 
> decapsulation function.

Thank you for the input.

I encounter this exact issue when I did an experiment for DHKEM(X25519,
HKDF-SHA256).  Currently, it computes public key from secret key.

My experiment is here:

    https://dev.gnupg.org/source/libgcrypt/history/gniibe%252Fkem2/

This is the branch on top of master.

Last month, I created the gniibe/kem branch on top of 1.10 branch.  I
need HKDF function for the DHKEM, so, I did again on top of master.
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