KNOWN BUGS:
- There is a small memory leak that appears to be occuring with token allocation.  I haven't been able to pinpoint this yet.
- Typing "OLD" at the new/old prompt upon startup when no existing BASIC source files are present results in an infinite loop, whereby the user is prompted for a filename that doesn't exist.  To get out of this, just hit Ctrl-C and start over.
- The parser is very slow, and has a mixture of full and arm's length recursion.  This could be tightened up quite a bit.
- The parser accepts FOR loops on subscripted variables (it's not supposed to).  This will probably throw an exception in practice.
- Since the lexer ignores whitespaces, LIST provides output that has no spaces in it.  It's legible but ugly.
- No bounds checking is done on constants, meaning that the original 9-character requirement is not enforced.  The only bounds checking is currently through the implementation of numeric values, which just relies upon a standard C float variable.
- Because PRINT always prints to the next tab stop, the command prompt after running a program will usually reappear on a tab stop, instead of on a new line.