Add source amalgamation

The amalgamated source concatenates all sources
to a file janet.c which can be used for
embedding janet, much in the same way as sqlite
or mongoose.

src/core/vm.c

@@ -20,12 +20,14 @@
 * IN THE SOFTWARE.
 */
 
+#ifndef JANET_AMALG
 #include <janet/janet.h>
 #include "state.h"
 #include "fiber.h"
 #include "gc.h"
 #include "symcache.h"
 #include "util.h"
+#endif
 
 /* VM state */
 JANET_THREAD_LOCAL JanetTable *janet_vm_registry;