Refactor embedder interface for conservative GC

Now users don't have to #ifdef on conservative vs precise tracing; it's
just a generic embedder concern.

simple-roots-api.h

@@ -0,0 +1,25 @@
+#ifndef SIMPLE_ROOTS_API_H
+#define SIMPLE_ROOTS_API_H
+
+#include "gc-config.h"
+#include "simple-roots-types.h"
+
+#define HANDLE_TO(T) union { T* v; struct handle handle; }
+#define HANDLE_REF(h) h.v
+#define HANDLE_SET(h,val) do { h.v = val; } while (0)
+#define PUSH_HANDLE(cx, h) push_handle(&(cx)->roots.roots, &h.handle)
+#define POP_HANDLE(cx) pop_handle(&(cx)->roots.roots)
+
+static inline void push_handle(struct handle **roots, struct handle *handle) {
+  if (GC_PRECISE_ROOTS) {
+    handle->next = *roots;
+    *roots = handle;
+  }
+}
+
+static inline void pop_handle(struct handle **roots) {
+  if (GC_PRECISE_ROOTS)
+    *roots = (*roots)->next;
+}
+
+#endif // SIMPLE_ROOTS_API_H