Fix #2725 by calling load_context/1 in the unspecified branch of strip_module/3

This fixes #2725, by making it so that `strip_module(Pred, M, P), call(M:P)`
doesn't throw an `instanciation_error` when `Pred` isn't in the form `module:predicate`.

Now, `strip_module(hello, M, P)` will call `load_context(M)`, which unifies `M`
with the topmost module (or `user`).

Two new test cases are added: issue2725.pl, which tests the minimal case id(X) --> X.
and the strip_module(P, M, _), call(M:P) scenario, and module_resolution,
which tests the behavior of strip_module in a few scenarios.

tests/scryer/issues.rs

@@ -33,3 +33,13 @@ fn call_qualification() {
 fn load_context_unreachable() {
     load_module_test("tests-pl/load-context-unreachable.pl", "");
 }
+
+// Issue #2725: A dcg of the form `id(X) --> X.` would previously trigger an instantiation
+// error, as it would call `strip_module(X, M, P)` and later `call(M:P)`,
+// but `strip_module` left `M` uninstanciated if the `module:` prefix was unspecified.
+#[serial]
+#[test]
+#[cfg_attr(miri, ignore = "it takes too long to run")]
+fn issue2725_dcg_without_module() {
+    load_module_test("tests-pl/issue2725.pl", "");
+}