Use a different approach to reading from buffers

Use a different approach to reading from buffers

We now store INPUT-BUFFER as a global variable. Words like S" and READ-WORD
automatically use this INPUT-BUFFER if it's available.

Our program currently segfaults on startup, which is probably because of at
least one of the following:

1. We don't handle the case where an input buffer ends in whitespace in
   INTERPRET-STRING.
2. We don't do anything to resume reading from user input when the buffer has
   been exhausted.