added a TODO list

TODO

@@ -0,0 +1,22 @@
+* Guardians:
+  - clean up after file ports are dead by flushing/closing the
+    underlying file handle.
+
+* Interrupts:
+  - pcb should have an engine-counter field that's decremented on
+    every procedure call.
+  - an engine-expired procedure must be called when the counter goes
+    negative.  It should capture the current continuation and
+    reschedule the process.
+  - figure out what to do when a process gets interrupted while
+    reading from a file.
+  - Implement [CTRL-C] keyboard interruption with the ability to
+    resume the computation, reset to the repl, or terminate the
+    program (at least for now).
+
+* FFI: 
+  - provide a general mechanism for calling foreign procedures
+    without having to extend the VM.
+    (foreign-call void: FOO int32: x char*: y ptr: z)
+  - may dump the bignum stuff in the VM and move it to scheme.
+