Added Extended Hash-Based Signatures (XMSS)

[1] XMSS: Extended Hash-Based Signatures,
    draft-itrf-cfrg-xmss-hash-based-signatures-06
    Release: July 2016.
    https://datatracker.ietf.org/doc/
    draft-irtf-cfrg-xmss-hash-based-signatures/?include_text=1

Provides XMSS_PublicKey and XMSS_PrivateKey classes as well as implementations
for the Botan interfaces PK_Ops::Signature and PK_Ops::Verification. XMSS has
been integrated into the Botan test bench, signature generation and verification
can be tested independently by invoking "botan-test xmss_sign" and
"botan-test xmss_verify"

- Some headers that are not required to be exposed to users of the library have
  to be declared as public in `info.txt`. Declaring those headers private will
  cause the amalgamation build to fail. The following headers have been
  declared public inside `info.txt`, even though they are only intended for
  internal use:
    * atomic.h
    * xmss_hash.h
    * xmss_index_registry.h
    * xmss_address.h
    * xmss_common_ops.h
    * xmss_tools.h
    * xmss_wots_parameters.h
    * xmss_wots_privatekey.h
    * xmss_wots_publickey.h

- XMSS_Verification_Operation Requires the "randomness" parameter out of the
  XMSS signature. "Randomness" is part of the prefix that is hashed *before*
  the message. Since the signature is unknown till sign() is called, all
  message content has to be buffered. For large messages this can be
  inconvenient or impossible.

  **Possible solution**: Change PK_Ops::Verification interface to take
  the signature as constructor argument, and provide a setter method to be able
  to update reuse the instance on multiple signatures. Make sign a parameterless
  member call. This solution requires interface changes in botan.

  **Suggested workaround** for signing large messages is to not sign the message
  itself, but to precompute the message hash manually using Botan::HashFunctio
  and sign the message hash instead of the message itself.

- Some of the available test vectors for the XMSS signature verification have
  been commented out in order to reduce testbench runtime.

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