small refactor and cleanup

tests/io_read_file.sl

@@ -0,0 +1,36 @@
+import ../stdlib/stdlib.sl
+import ../stdlib/io.sl
+
+: main
+    "/tmp/l2_read_file_test.txt"
+    "read_file works\n"
+    write_file drop
+
+    "/tmp/l2_read_file_test.txt" # (addr len)
+    read_file                 # (file_addr file_len)
+    dup 0 > if                # if file_len > 0, success
+        write_buf             # print file contents (file_len file_addr)
+        0
+        exit
+    end
+    dup -2 == if              # open() failed
+        drop
+        "open() failed: errno=" puts
+        swap puti cr
+        exit
+    end
+    dup -1 == if              # fstat() failed
+        drop
+        "fstat() failed: errno=" puts
+        swap puti cr
+        exit
+    end
+    dup -3 == if              # mmap() failed
+        drop
+        "mmap() failed" puts
+        exit
+    end
+    "unknown read_file failure" puts
+    dup                       # file_len file_len file_addr
+    exit                       # Exit with returned file_len as the program exit code (debug)
+;