ISO core `ensure_loaded/1` directive is not recognized

[rotu]

Using the ensure_loaded/1 directive in Prolog text results in an error at load time:

e.g.

$ cat .\scratch.pl
:- ensure_loaded(some_file).
$ scryer-prolog.exe .\scratch.pl
   error(domain_error(directive,ensure_loaded/1),load/1).

This directive is defined in ISO/IEC 13211-1:

7.4.2.8 ensure_loaded/1

A directive ensure_loaded(P_text) specifies that the Prolog text being prepared for execution shall include the Prolog text denoted by P_text where P_text is an implementation defined ground term designating a Prolog text unit

When multiple directives ensure_loaded(P_text) exist for the same Prolog text, that Prolog text is included in the Prolog text prepared for execution only once. The position where it is included is implementation defined.

[triska]

Related: #634.