🚸 Add operator[] to messages

Problem:
- Messages support `get` and `set` but not a natural indexing syntax.

Solution:
- Add `operator[]` to message, so that the follow work:

```c+++
msg["a"_field] = 42;
auto x = msg["a"_field];
```

Author: elbeno

include/msg/message.hpp

@@ -344,11 +344,37 @@ template <stdx::ct_string Name, typename Access, typename T> struct msg_base {
     [[nodiscard]] constexpr auto get(auto f) const {
         return Access::get(as_derived().data(), f);
     }
+
     constexpr auto set(auto... fs) -> void {
         Access::set(as_derived().data(), fs...);
     }
     constexpr auto set() -> void {}
 
+    template <stdx::ct_string N, typename B> struct proxy {
+        // NOLINTNEXTLINE(cppcoreguidelines-avoid-const-or-ref-data-members)
+        B &b;
+
+        // NOLINTNEXTLINE(misc-unconventional-assign-operator)
+        constexpr auto operator=(auto val) const && -> void {
+            b.set(field_name<N>{} = val);
+        }
+
+        using V = decltype(b.get(std::declval<field_name<N>>()));
+
+        // NOLINTNEXTLINE(google-explicit-constructor)
+        constexpr operator V() const && { return b.get(field_name<N>{}); }
+    };
+
+    template <stdx::ct_string N>
+    [[nodiscard]] constexpr auto operator[](field_name<N> f) const {
+        return get(f);
+    }
+    template <stdx::ct_string N>
+    [[nodiscard]] constexpr auto
+    operator[](field_name<N>) LIFETIMEBOUND->proxy<N, msg_base> {
+        return {*this};
+    }
+
     [[nodiscard]] constexpr auto describe() const {
         return Access::describe(as_derived().data());
     }

test/msg/message.cpp

@@ -792,3 +792,17 @@ TEST_CASE("pack appends environments", "[message]") {
     using defn = pack<"defn", std::uint8_t, m1, m2>;
     STATIC_REQUIRE(custom(defn::env_t{}) == 18);
 }
+
+TEST_CASE("read indexing operator on message", "[message]") {
+    test_msg const msg{};
+    CHECK(0x80 == msg["id"_field]);
+}
+
+TEST_CASE("write indexing operator on message", "[message]") {
+    test_msg msg{};
+    bool before = 0 == msg["f1"_field];
+    CHECK(before);
+    msg["f1"_field] = 0xba11;
+    bool after = 0xba11 == msg["f1"_field];
+    CHECK(after);
+}