support cstr as an ffi return type and do some more restructuring

tests-pl/ffi_cstr.pl

@@ -0,0 +1,16 @@
+:- use_module(library(os)).
+:- use_module(library(ffi)).
+
+test :- 
+    read(Body),
+    term_variables(Body, [LIB]),
+    Body,
+    use_foreign_module(LIB, [
+        'ffi_cstr_len'([cstr], u64),
+        'ffi_example_cstr'([], cstr)
+    ]),
+    ffi:'ffi_cstr_len'("Scryer Prolog", Len),
+    ffi:'ffi_example_cstr'(Str),
+    write((Len-Str)).
+
+:- initialization(test).