Forth in Rust.

FRUSTRATION has got a foot standing on its own tail because writing a
monolithic outer interpreter in a high level language makes it really
annoying to monkey with the functioning of the interpreter from within
the language it's interpreting.  PARSE/WORD and the input stream
handling was the first place this became obvious.  This design is a
dead end.  The path forward would be stripping it back to primitives
and rewriting the outer interpreter in Forth.

Here are things it can do today:

Print some terms of the fibonacci sequence:
```
: over  >r dup r> swap ;
: fib  recursive  r> drop  over + swap  dup .  dup 144 - ? fib ;
: fib  1 0 fib ;

fib
1 1 2 3 5 8 13 21 34 55 89 144  ok
```

Compute the number of cans in a triangular stack of height n.
For example a stack of height 3 contains 6 cans.
```
  x
 x x 
x x x
```

```
variable cans
: c  recursive  r> drop  dup cans @ + cans !  1 -  dup ?  c ;
: c  c ;
: can-stack  0 cans !  c  cans @ ;

3 can-stack .
6  ok
10 can-stack .
55  ok
```