Retro's memory is directly accessible via @ and !. This is used for all functions and
data structures. A variable, heap, points to the next free address. allot can be used
to allocate or free memory. The amount of memory varies by the runtime, and can be
accessed via the memory variable.

( display total memory available )
@memory putn

( display unused memory )
@memory here - putn

( display next free address )
here putn

( allocate 1000 cells )
1000 allot

( free 500 cells )
-500 allot
