add ffi:{allocate,read_ptr,deallocate}

tests-pl/ffi_heap.pl

@@ -0,0 +1,19 @@
+:- use_module(library(os)).
+:- use_module(library(ffi)).
+
+init :-
+    read(Body),
+    term_variables(Body, [LIB]),
+    Body,
+    use_foreign_module(LIB, [
+        'ffi_set_u64'([ptr], void)
+    ]).
+
+test :-
+    ffi:allocate(rust, u64, 0, Ptr),
+    ffi:'ffi_set_u64'(Ptr),
+    ffi:read_ptr(u64, Ptr, Val),
+    ffi:deallocate(rust, u64, Ptr),
+    write((Val)).
+
+:- initialization((init,test)).