proxy: add BuildList helper and dedup set/vector build handlers

Author: ViniciusCestarii

include/mp/proxy-types.h

@@ -280,6 +280,17 @@ struct ListOutput<::capnp::List<T, kind>>
     // clang-format on
 };
 
+template <typename LocalType, typename Value, typename Output>
+void BuildList(TypeList<LocalType>, InvokeContext& invoke_context, Output&& output, Value&& value)
+{
+    auto list = output.init(value.size());
+    size_t i = 0;
+    for (const auto& elem : value) {
+        BuildField(TypeList<LocalType>(), invoke_context, ListOutput<typename decltype(list)::Builds>(list, i), elem);
+        ++i;
+    }
+}
+
 template <typename LocalType, typename Value, typename Output>
 void CustomBuildField(TypeList<LocalType>, Priority<0>, InvokeContext& invoke_context, Value&& value, Output&& output)
 {

include/mp/type-set.h

@@ -16,13 +16,7 @@ void CustomBuildField(TypeList<std::set<LocalType>>,
     Value&& value,
     Output&& output)
 {
-    // FIXME dededup with vector handler above
-    auto list = output.init(value.size());
-    size_t i = 0;
-    for (const auto& elem : value) {
-        BuildField(TypeList<LocalType>(), invoke_context, ListOutput<typename decltype(list)::Builds>(list, i), elem);
-        ++i;
-    }
+    BuildList(TypeList<LocalType>(), invoke_context, output, value);
 }
 
 template <typename LocalType, typename Input, typename ReadDest>

include/mp/type-vector.h

@@ -16,12 +16,7 @@ void CustomBuildField(TypeList<std::vector<LocalType>>,
     Value&& value,
     Output&& output)
 {
-    // FIXME dedup with set handler below
-    auto list = output.init(value.size());
-    size_t i = 0;
-    for (auto it = value.begin(); it != value.end(); ++it, ++i) {
-        BuildField(TypeList<LocalType>(), invoke_context, ListOutput<typename decltype(list)::Builds>(list, i), *it);
-    }
+    BuildList(TypeList<LocalType>(), invoke_context, output, value);
 }
 
 inline static bool BuildPrimitive(InvokeContext& invoke_context, std::vector<bool>::const_reference value, TypeList<bool>)