add memberchk/2 built-in predicate

[s-webber]

memberchk(X, Y) returns true if X is an element of the list Y

(Unlike "member(X, Y)", no attempt is made to re-evaluate during backtracking.)

Example use:

?- memberchk(a,[a,b,c]).
true.

?- memberchk(b,[a,b,c]).
true.

?- memberchk(c,[a,b,c]).
true.

?- memberchk(d,[a,b,c]).
false.

?- memberchk(a,[a,b,a,c]).
true.

?- memberchk(X,[a,b,a,c]).
X = a.