Refactor for cleanliness

Move files into separate folders for ease of reading, include source
directory so we can use angle bracket includes, adjust build system to
make directories for objects

main.c

@@ -13,12 +13,10 @@
  * Description: Entrypoint
  */
 
-#include "./base.h"
-#include "./lisp.h"
-#include "./memory.h"
-#include "./reader.h"
-#include "./sv.h"
-#include "./vec.h"
+#include <lib/sv.h>
+#include <lib/vec.h>
+#include <lisp/lisp.h>
+#include <lisp/reader.h>
 
 #include <assert.h>
 #include <ctype.h>
@@ -27,6 +25,15 @@
 #include <stdio.h>
 #include <string.h>
 
+void usage(char *prog_name, FILE *fp)
+{
+  fprintf(fp,
+          "Usage: %s [FILE]\n"
+          "Interpret FILE as scheme in the OATS interpreter.\n"
+          "\t[FILE]: name of file\n",
+          prog_name);
+}
+
 int main(int argc, char *argv[])
 {
   int exit             = 0;
@@ -36,7 +43,10 @@ int main(int argc, char *argv[])
   if (argc > 1)
     filename = argv[1];
   else
-    filename = "./r7rs-tests.scm";
+  {
+    usage(argv[0], stderr);
+    return 1;
+  }
 
   FILE *fp = fopen(filename, "r");
   if (!fp)