Split vm/runtime.c into two files

struct.c is used for the general construction, inspection and deletion
of the virtual machine structure (vm_t).  runtime defines the
execution routines, error enum, lookup tables, etc.

vm/main.c

@@ -14,9 +14,11 @@
 #include <stdio.h>
 #include <string.h>
 
-#include "./runtime.h"
 #include <lib/inst.h>
 
+#include "./runtime.h"
+#include "./struct.h"
+
 void usage(const char *program_name, FILE *out)
 {
   fprintf(out,