Simul efun: set_variable - set a value in an alist

Synopsis:
	mixed set_variable(mixed* pool, string variable, mixed value)

Description:
	Put or remove a variable into a pool of variables, actually
	a handler to interact with any 2-dimensional alists.

	A pool is a 2-dim alist, it looks typically like this:
	({
		({ "varname1", "varname2" }),
		({ "contents1", "contents2" })
	})

	But the structure doesn't really matter, since you should not need
	to act upon it directly. This function will do for you:

	pool = set_variable(0, "blah", 23);
		.. generates a variable pool containing a variable "blah"
		associated to the integer 23.

	pool = set_variable(pool, "blah");
		.. removes the variable "blah" from the pool.

	Hold your breath, this function might already be obsolete
	as soon as mappings are introduced, *however* the syntax will
	be supported for compatibility, just the internal structure
	of the pools will change from alist to mapping.

Note:
	Kiri says: "I don't trust those simulated efuns and real efuns
	messing with alists! 'assoc' is not clean - at least not clean
	as simple replacement for member_array!"

See also: lib/query_variable, efun/order_alist, efun/assoc, LPC/alists