For #469 - Add support for C++ and mixed C/C++

WIP and for native modules. Required a few changes to headers and
some changes to JPM.

test/install/project.janet

@@ -9,6 +9,10 @@
     :name "testmod2"
     :source @["testmod2.c"])
 
+(declare-native
+    :name "testmod3"
+    :source @["testmod3.cpp"])
+
 (declare-executable
   :name "testexec"
   :entry "testexec.janet")

test/install/testexec.janet

@@ -1,6 +1,7 @@
 (use build/testmod)
 (use build/testmod2)
+(use build/testmod3)
 
 (defn main [&]
   (print "Hello from executable!")
-  (print (+ (get5) (get6))))
+  (print (+ (get5) (get6) (get7))))

test/install/testmod3.cpp

@@ -0,0 +1,42 @@
+/*
+* Copyright (c) 2020 Calvin Rose and contributors
+*
+* Permission is hereby granted, free of charge, to any person obtaining a copy
+* of this software and associated documentation files (the "Software"), to
+* deal in the Software without restriction, including without limitation the
+* rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+* sell copies of the Software, and to permit persons to whom the Software is
+* furnished to do so, subject to the following conditions:
+*
+* The above copyright notice and this permission notice shall be included in
+* all copies or substantial portions of the Software.
+*
+* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+* IN THE SOFTWARE.
+*/
+
+/* A very simple native module */
+
+#include <janet.h>
+#include <iostream>
+
+static Janet cfun_get_seven(int32_t argc, Janet *argv) {
+    (void) argv;
+    janet_fixarity(argc, 0);
+    std::cout << "Hello!" << std::endl;
+    return janet_wrap_number(7.0);
+}
+
+static const JanetReg array_cfuns[] = {
+    {"get7", cfun_get_seven, NULL},
+    {NULL, NULL, NULL}
+};
+
+JANET_MODULE_ENTRY(JanetTable *env) {
+    janet_cfuns(env, NULL, array_cfuns);
+}