Add X25519 and Ed25519 support.

(Ed25519 support is disabled when |OPENSSL_SMALL| is defined.)

libcrypto.a sizes:

x86-64 -O3 -march=native: +78012 (1584902 → 1662914)
x86-64 -O3 -march=native -DOPENSSL_SMALL: +10596 (1356206 → 1366802)
Android armv7 Thumb -O2 -DOPENSSL_SMALL: +13132 (1258462 → 1271594)

Change-Id: I6a7e64d481e4ce4daa7d5057578081358746cfb9
Reviewed-on: https://boringssl-review.googlesource.com/6497
Reviewed-by: Adam Langley <agl@google.com>

util/doc.config

diff --git a/util/doc.config b/util/doc.config
index 77498ee..cbee543 100644
--- a/util/doc.config
+++ b/util/doc.config
@@ -24,6 +24,7 @@
       "include/openssl/aes.h",
       "include/openssl/bn.h",
       "include/openssl/cmac.h",
+      "include/openssl/curve25519.h",
       "include/openssl/des.h",
       "include/openssl/dh.h",
       "include/openssl/dsa.h",