Add initializer_list constructor to json_arrayt

This new constructor facilitates the construction of instances of
`json_arrayt` which are const, because it gives a tidy way to construct
an entire `json_arrayt`.

Author: thomasspriggs

src/util/json.h

@@ -165,6 +165,11 @@ class json_arrayt:public jsont
   {
   }
 
+  explicit json_arrayt(std::initializer_list<jsont> initializer_list)
+    : json_arrayt{arrayt{initializer_list}}
+  {
+  }
+
   void resize(std::size_t size)
   {
     array.resize(size);

unit/Makefile

@@ -51,6 +51,7 @@ SRC += analyses/ai/ai.cpp \
        util/graph.cpp \
        util/irep.cpp \
        util/irep_sharing.cpp \
+       util/json_array.cpp \
        util/json_object.cpp \
        util/memory_info.cpp \
        util/message.cpp \

unit/util/json_array.cpp

@@ -0,0 +1,35 @@
+/*******************************************************************\
+
+Module: Catch tests for json_arrayt
+
+Author: Diffblue Ltd.
+
+\*******************************************************************/
+
+#include <testing-utils/catch.hpp>
+#include <util/json.h>
+
+SCENARIO(
+  "Test that json_arrayt can be constructed from an initializer list.",
+  "[core][util][json]")
+{
+  GIVEN("A json_arrayt constructed from an initializer list.")
+  {
+    const json_arrayt array{
+      json_stringt{"one"}, json_numbert{"2"}, json_stringt{"three"}};
+    THEN("The elements of the `json_arrayt` match the initialiser list.")
+    {
+      auto it = array.begin();
+      REQUIRE(it->kind == jsont::kindt::J_STRING);
+      REQUIRE(it->value == "one");
+      ++it;
+      REQUIRE(it->kind == jsont::kindt::J_NUMBER);
+      REQUIRE(it->value == "2");
+      ++it;
+      REQUIRE(it->kind == jsont::kindt::J_STRING);
+      REQUIRE(it->value == "three");
+      ++it;
+      REQUIRE(it == array.end());
+    }
+  }
+}