libgap API: expose minimal interface to garbage collector (#4215)

Add libgap interfaces to the garbage collector.  As discussed in #3030
these are probably the minimal interfaces that need to be exposed, and
are compatible with all current GC implementations.

As suggested at #3030 (comment)
GAP_CollectBags only takes a <full> argument, but not <size>

src/libgap-api.c

@@ -67,6 +67,20 @@ void GAP_Initialize(int              argc,
 }
 
 
+////
+//// Garbage collector interface
+////
+void GAP_MarkBag(Obj obj)
+{
+    MarkBag(obj);
+}
+
+void GAP_CollectBags(BOOL full)
+{
+    CollectBags(0, full);
+}
+
+
 ////
 //// program evaluation and execution
 ////

src/libgap-api.h

@@ -161,6 +161,26 @@ void GAP_Initialize(int              argc,
                     int              handleSignals);
 
 
+////
+//// Garbage collector
+////
+
+// Manual management of the GAP garbage collector: for cases where you want
+// a GAP object to be long-lived beyond the context of the stack frame where
+// it was created, it is necessary to call GAP_MarkBag on the object when
+// the garbage collector is run by the markBagsCallback function passed to
+// GAP_Initialize.
+void GAP_MarkBag(Obj obj);
+
+// Manually run the garbage collector.
+// A <full> collection checks all previously allocated objects, including those
+// that have survived at least one previous garbage collection.
+// A partial collection will attempt to clean up only recently allocated
+// objects which have not been garbage-collected yet, and is hence normally
+// a faster operation.
+void GAP_CollectBags(BOOL full);
+
+
 ////
 //// program evaluation and execution
 ////