ISO: Implement the include/1 directive

ISO: Implement the include/1 directive

Quoting from the standard:

    7.4.2.7 include/1

    If F is an implementation defined ground term designating
    a Prolog text unit, then Prolog text P1 which contains
    a directive include(F) is identical to a Prolog text P2
    obtained by replacing the directive include(F) in P1 by
    the Prolog text denoted by F.

Example:

    :- include("hello.pl").

This addresses #583 and #634.