state: Implement CREATE address without RLP lib

Remove Host's dependency on RLP library by re-implementing
`compute_create_address()`.

Closes #783.

test/state/host.cpp

@@ -4,7 +4,6 @@
 
 #include "host.hpp"
 #include "precompiles.hpp"
-#include "rlp.hpp"
 #include <evmone/constants.hpp>
 #include <evmone/eof.hpp>
 
@@ -179,11 +178,35 @@ bool Host::selfdestruct(const address& addr, const address& beneficiary) noexcep
 
 address compute_create_address(const address& sender, uint64_t sender_nonce) noexcept
 {
-    // TODO: Compute CREATE address without using RLP library.
-    const auto rlp_list = rlp::encode_tuple(sender, sender_nonce);
-    const auto base_hash = keccak256(rlp_list);
+    static constexpr auto RLP_STR_BASE = 0x80;
+    static constexpr auto RLP_LIST_BASE = 0xc0;
+    static constexpr auto ADDRESS_SIZE = sizeof(sender);
+    static constexpr std::ptrdiff_t MAX_NONCE_SIZE = sizeof(sender_nonce);
+
+    uint8_t buffer[ADDRESS_SIZE + MAX_NONCE_SIZE + 3];  // 3 for RLP prefix bytes.
+    auto p = &buffer[1];                                // Skip RLP list prefix for now.
+    *p++ = RLP_STR_BASE + ADDRESS_SIZE;                 // Set RLP string prefix for address.
+    p = std::copy_n(sender.bytes, ADDRESS_SIZE, p);
+
+    if (sender_nonce < RLP_STR_BASE)  // Short integer encoding including 0 as empty string (0x80).
+    {
+        *p++ = sender_nonce != 0 ? static_cast<uint8_t>(sender_nonce) : RLP_STR_BASE;
+    }
+    else  // Prefixed integer encoding.
+    {
+        // TODO: bit_width returns int after [LWG 3656](https://cplusplus.github.io/LWG/issue3656).
+        const auto num_nonzero_bytes = static_cast<int>((std::bit_width(sender_nonce) + 7) / 8);
+        *p++ = static_cast<uint8_t>(RLP_STR_BASE + num_nonzero_bytes);
+        intx::be::unsafe::store(p, sender_nonce);
+        p = std::shift_left(p, p + MAX_NONCE_SIZE, MAX_NONCE_SIZE - num_nonzero_bytes);
+    }
+
+    const auto total_size = static_cast<size_t>(p - buffer);
+    buffer[0] = static_cast<uint8_t>(RLP_LIST_BASE + (total_size - 1));  // Set the RLP list prefix.
+
+    const auto base_hash = keccak256({buffer, total_size});
     address addr;
-    std::copy_n(&base_hash.bytes[sizeof(base_hash) - sizeof(addr)], sizeof(addr), addr.bytes);
+    std::copy_n(&base_hash.bytes[sizeof(base_hash) - ADDRESS_SIZE], ADDRESS_SIZE, addr.bytes);
     return addr;
 }