C++: Fix basic_string<unsigned char> (#712)

The `std::basic_string<unsigned char>` is not standard compliant because
it relies on non-standard specialization `std::char_traits<unsigned
char>`.

The later has been deprecated in libc++ and will be removed in version
19.

Define our own traits `byte_traits<unsigned char>` and define the types
for storing bytes as:
`std::basic_string<unsigned char, byte_traits<unsigned char>`.

include/evmc/bytes.hpp

@@ -0,0 +1,93 @@
+// EVMC: Ethereum Client-VM Connector API.
+// Copyright 2024 The EVMC Authors.
+// Licensed under the Apache License, Version 2.0.
+#pragma once
+
+#include <algorithm>
+#include <cstring>
+#include <string>
+#include <string_view>
+
+namespace evmc
+{
+/// The char traits for byte-like types.
+///
+/// See: https://en.cppreference.com/w/cpp/string/char_traits.
+template <typename T>
+struct byte_traits : std::char_traits<char>
+{
+    static_assert(sizeof(T) == 1, "type must be a byte");
+
+    using char_type = T;  ///< The byte type.
+
+    /// Assigns c2 to c1.
+    static constexpr void assign(char_type& c1, const char_type& c2) { c1 = c2; }
+
+    /// Assigns value to each byte in [ptr, ptr+count).
+    static constexpr char_type* assign(char_type* ptr, std::size_t count, char_type value)
+    {
+        std::fill_n(ptr, count, value);
+        return ptr;
+    }
+
+    /// Returns true if bytes are equal.
+    static constexpr bool eq(char_type a, char_type b) { return a == b; }
+
+    /// Returns true if byte a is less than byte b.
+    static constexpr bool lt(char_type a, char_type b) { return a < b; }
+
+    /// Copies count bytes from src to dest. Performs correctly even if ranges overlap.
+    static constexpr char_type* move(char_type* dest, const char_type* src, std::size_t count)
+    {
+        if (dest < src)
+            std::copy_n(src, count, dest);
+        else if (src < dest)
+            std::copy_backward(src, src + count, dest + count);
+        return dest;
+    }
+
+    /// Copies count bytes from src to dest. The ranges must not overlap.
+    static constexpr char_type* copy(char_type* dest, const char_type* src, std::size_t count)
+    {
+        std::copy_n(src, count, dest);
+        return dest;
+    }
+
+    /// Compares lexicographically the bytes in two ranges of equal length.
+    static constexpr int compare(const char_type* a, const char_type* b, std::size_t count)
+    {
+        for (; count != 0; --count, ++a, ++b)
+        {
+            if (lt(*a, *b))
+                return -1;
+            if (lt(*b, *a))
+                return 1;
+        }
+        return 0;
+    }
+
+    /// Returns the length of a null-terminated byte string.
+    // TODO: Not constexpr
+    static std::size_t length(const char_type* s)
+    {
+        return std::strlen(reinterpret_cast<const char*>(s));
+    }
+
+    /// Finds the value in the range of bytes and returns the pointer to the first occurrence
+    /// or nullptr if not found.
+    static constexpr const char_type* find(const char_type* s,
+                                           std::size_t count,
+                                           const char_type& value)
+    {
+        const auto end = s + count;
+        const auto p = std::find(s, end, value);
+        return p != end ? p : nullptr;
+    }
+};
+
+/// String of unsigned chars representing bytes.
+using bytes = std::basic_string<unsigned char, byte_traits<unsigned char>>;
+
+/// String view of unsigned chars representing bytes.
+using bytes_view = std::basic_string_view<unsigned char, byte_traits<unsigned char>>;
+}  // namespace evmc

include/evmc/evmc.hpp

@@ -20,9 +20,6 @@ static_assert(EVMC_LATEST_STABLE_REVISION <= EVMC_MAX_REVISION,
 /// @ingroup cpp
 namespace evmc
 {
-/// String view of uint8_t chars.
-using bytes_view = std::basic_string_view<uint8_t>;
-
 /// The big-endian 160-bit hash suitable for keeping an Ethereum address.
 ///
 /// This type wraps C ::evmc_address to make sure objects of this type are always initialized.

include/evmc/hex.hpp

@@ -3,6 +3,7 @@
 // Licensed under the Apache License, Version 2.0.
 #pragma once
 
+#include <evmc/bytes.hpp>
 #include <evmc/filter_iterator.hpp>
 #include <cstdint>
 #include <optional>
@@ -11,13 +12,6 @@
 
 namespace evmc
 {
-/// String of uint8_t chars.
-using bytes = std::basic_string<uint8_t>;
-
-/// String view of uint8_t chars.
-using bytes_view = std::basic_string_view<uint8_t>;
-
-
 /// Encode a byte to a hex string.
 inline std::string hex(uint8_t b) noexcept
 {

include/evmc/mocked_host.hpp

@@ -12,9 +12,6 @@
 
 namespace evmc
 {
-/// The string of bytes.
-using bytes = std::basic_string<uint8_t>;
-
 /// Extended value (with original value and access flag) for account storage.
 struct StorageValue
 {