Merge #558: Add schnorrsig module which implements BIP-340 compliant signatures

Merge #558: Add schnorrsig module which implements BIP-340 compliant signatures

f431b3f28ac95a3645ad5a6dc96b878fa30a1de3 valgrind_ctime_test: Add schnorrsig_sign (Jonas Nick)
16ffa9d97cef93f49544b016339c107882f9a1c3 schnorrsig: Add taproot test case (Jonas Nick)
8dfd53ee3fa059562483d1867815f78b9e00d947 schnorrsig: Add benchmark for sign and verify (Jonas Nick)
4e43520026f5bcd182d21f0759bac159ef47bb62 schnorrsig: Add BIP-340 compatible signing and verification (Jonas Nick)
7332d2db6b62fda851f9ed8adbfda187a875b84e schnorrsig: Add BIP-340 nonce function (Jonas Nick)
7a703fd97db0161bae07ef84513ddde6e0d27353 schnorrsig: Init empty experimental module (Jonas Nick)
eabd9bc46a31c0da6db6d88840eadbe9006447b1 Allow initializing tagged sha256 (Jonas Nick)
6fcb5b845d2832ce019d60507033f74426290768 extrakeys: Add keypair_xonly_tweak_add (Jonas Nick)
58254463f9a2e96d893157a341c9953c440fdf60 extrakeys: Add keypair struct with create, pub and pub_xonly (Jonas Nick)
f0010349b876bc6b3f0a6ec6c8bad0b12ca17b51 Separate helper functions for pubkey_create and seckey_tweak_add (Jonas Nick)
910d9c284c33b77774a9316d4524f313357d441c extrakeys: Add xonly_pubkey_tweak_add & xonly_pubkey_tweak_add_test (Jonas Nick)
176bfb1110147b5dca1834ea071acc846fb1cab3 Separate helper function for ec_pubkey_tweak_add (Jonas Nick)
4cd2ee474d178bd1b5602486104db346a7562c67 extrakeys: Add xonly_pubkey with serialize, parse and from_pubkey (Jonas Nick)
47e6618e11813cfabe91f0909ca031f960cb7dd4 extrakeys: Init empty experimental module (Jonas Nick)
3e08b02e2a78f2a1fc457efab665db8ab8085373 Make the secp256k1_declassify argument constant (Jonas Nick)

Pull request description:

  This PR implements signing, verification and batch verification as described in [BIP-340](https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki) in an experimental module named `schnorrsig`. It includes the test vectors and a benchmarking tool.
  This PR also adds a module `extrakeys` that allows [BIP-341](https://github.com/bitcoin/bips/blob/master/bip-0341.mediawiki)-style key tweaking.

  (Adding ChaCha20 as a CSPRNG and batch verification was moved to PR #760).

  In order to enable the module run `./configure` with `--enable-experimental --enable-module-schnorrsig`.

  Based on apoelstra's work.

ACKs for top commit:
  gmaxwell:
    ACK f431b3f28ac95a3645ad5a6dc96b878fa30a1de3  (exactly matches the previous post-fixup version which I have already reviewed and tested)
  sipa:
    ACK f431b3f28ac95a3645ad5a6dc96b878fa30a1de3
  real-or-random:
    ACK f431b3f28ac95a3645ad5a6dc96b878fa30a1de3 careful code review

Tree-SHA512: e15e849c7bb65cdc5d7b1d6874678e275a71e4514de9d5432ec1700de3ba92aa9f381915813f4729057af152d90eea26aabb976ed297019c5767e59cf0bbc693