Bench test for EllSwift ECDH

Co-authored-by: Dhruv Mehta <856960+dhruv@users.noreply.github.com>

src/random.h

@@ -213,6 +213,9 @@ public:
     /** Generate random bytes. */
     std::vector<unsigned char> randbytes(size_t len);
 
+    /** Fill a byte Span with random bytes. */
+    void fillrand(Span<std::byte> output);
+
     /** Generate a random 32-bit integer. */
     uint32_t rand32() noexcept { return randbits(32); }