Undo PIMPL pattern in Environment (#127)

* resync single_include

* Undo PIMPL pattern in Environment

* Environment now supports copy construction and assignment
* Add test for copying Environment

Closes #126

test/unit.cpp

@@ -2,3 +2,33 @@
 
 #define CATCH_CONFIG_MAIN
 #include "catch/catch.hpp"
+#include "inja/inja.hpp"
+
+
+using json = nlohmann::json;
+
+
+TEST_CASE("copy-environment") {
+    inja::Environment env;
+    env.add_callback("double", 1, [](inja::Arguments& args) {
+        int number = args.at(0)->get<int>();
+        return 2 * number;
+    });
+
+    inja::Template t1 = env.parse("{{ double(2) }}");
+    env.include_template("tpl", t1);
+    std::string test_tpl = "{% include \"tpl\" %}";
+
+    REQUIRE(env.render(test_tpl, json()) == "4");
+
+    inja::Environment copy(env);
+    CHECK(copy.render(test_tpl, json()) == "4");
+
+    // overwrite template in source env
+    inja::Template t2 = env.parse("{{ double(4) }}");
+    env.include_template("tpl", t2);
+    REQUIRE(env.render(test_tpl, json()) == "8");
+
+    // template is unchanged in copy
+    CHECK(copy.render(test_tpl, json()) == "4");
+}