toptr improvements

toptr will return NULL (a new constant)if the variable doesn’t exist. == and != now support comparing pointers.

This is the new toptr.boo example which checks if the function exists before calling:

number f
file_open f "word.txt" "r"
string s
file_read f s
file_close f
pointer p
= p (toptr s)
if (!= p NULL) go nope
	call `p
:go
	print "NOPE!\n"
:nope
function foo
{
	print "foo!\n"
}
function bar
{
	print "bar!\n"
}
function baz
{
	print "baz!"
}