py/gc: Add gc_sweep_all() function to run all remaining finalisers.

This patch adds the gc_sweep_all() function which does a garbage collection
without tracing any root pointers, so frees all the memory, and most
importantly runs any remaining finalisers.

This helps primarily for soft reset: it will close any open files, any open
sockets, and help to get the system back to a clean state upon soft reset.

py/gc.c

@@ -362,6 +362,13 @@ void gc_collect_end(void) {
     GC_EXIT();
 }
 
+void gc_sweep_all(void) {
+    GC_ENTER();
+    MP_STATE_MEM(gc_lock_depth)++;
+    MP_STATE_MEM(gc_stack_overflow) = 0;
+    gc_collect_end();
+}
+
 void gc_info(gc_info_t *info) {
     GC_ENTER();
     info->total = MP_STATE_MEM(gc_pool_end) - MP_STATE_MEM(gc_pool_start);

py/gc.h

@@ -45,6 +45,9 @@ void gc_collect_start(void);
 void gc_collect_root(void **ptrs, size_t len);
 void gc_collect_end(void);
 
+// Use this function to sweep the whole heap and run all finalisers
+void gc_sweep_all(void);
+
 void *gc_alloc(size_t n_bytes, bool has_finaliser);
 void gc_free(void *ptr); // does not call finaliser
 size_t gc_nbytes(const void *ptr);