Add X25519 length constants.

Per request for named constants at
https://codereview.chromium.org/2457153003/diff/40001/net/quic/core/crypto/curve25519_key_exchange.cc#newcode27

The public one is named X25519_PUBLIC_VALUE_LEN rather than
X25519_PUBLIC_KEY_LEN for consistency with the rest of the
documentation. (Apparently we say "X25519 public value" and "Ed25519
public key".)

Change-Id: Idb76a0a9794f0f628c3b784e50a04fdb35bf3ee6
Reviewed-on: https://boringssl-review.googlesource.com/11941
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/curve25519.h

diff --git a/include/openssl/curve25519.h b/include/openssl/curve25519.h
index e9ba04d..8d73f33 100644
--- a/include/openssl/curve25519.h
+++ b/include/openssl/curve25519.h
@@ -33,6 +33,9 @@
  * sometimes referred to as “curve25519”, but “X25519” is a more precise name.
  * See http://cr.yp.to/ecdh.html and https://tools.ietf.org/html/rfc7748. */
 
+#define X25519_PRIVATE_KEY_LEN 32
+#define X25519_PUBLIC_VALUE_LEN 32
+
 /* X25519_keypair sets |out_public_value| and |out_private_key| to a freshly
  * generated, public–private key pair. */
 OPENSSL_EXPORT void X25519_keypair(uint8_t out_public_value[32],