Add fixups from upstream schnorrsig PR

f make helper functions static
f hash noncedata into nonce in nonce_function_bipschnorr
f expose nonce_function_bipschnorr
f fix undefined behavior when shifting an int 31 places
f add cplusplus ifdef to schnorrsig include file
f hash complete pubkey into batch seed
f chacha20 for bigendians
f add schnorrsig to travis
f show in configure if schnorrsig is enabled

include/secp256k1.h

@@ -523,6 +523,12 @@ SECP256K1_API int secp256k1_ecdsa_signature_normalize(
  */
 SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_rfc6979;
 
+/** An implementation of the nonce generation function as defined in BIP-schnorr.
+ * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
+ * extra entropy.
+ */
+SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_bipschnorr;
+
 /** A default safe nonce generation function (currently equal to secp256k1_nonce_function_rfc6979). */
 SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_default;
 

include/secp256k1_schnorrsig.h

@@ -1,6 +1,12 @@
 #ifndef SECP256K1_SCHNORRSIG_H
 #define SECP256K1_SCHNORRSIG_H
 
+#include "secp256k1.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** This module implements a variant of Schnorr signatures compliant with
  * BIP-schnorr
  * (https://github.com/sipa/bips/blob/bip-schnorr/bip-schnorr.mediawiki).
@@ -115,4 +121,9 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_schnorrsig_verify_batch
     const secp256k1_pubkey *const *pk,
     size_t n_sigs
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
+
+#ifdef __cplusplus
+}
 #endif
+
+#endif /* SECP256K1_SCHNORRSIG_H */