Fix LOAD-F-SPACE, pop 0 on empty stack, g and p instructions
LOAD-F-SPACE now stops reading from the string upon encountering a NUL
byte, all popping from the stack now uses the POP-STACK function, which
will return 0 if the stack is empty, g[et] and p[ut] instructions added
Wrapping
Wrapping is only toroidal for now, backtrack wrapping will be
implemented later
Instructions a-f