Add `rb_thread_current_scheduler()`.

internal/thread.h

@@ -37,9 +37,11 @@ void rb_mutex_allow_trap(VALUE self, int val);
 VALUE rb_uninterruptible(VALUE (*b_proc)(VALUE), VALUE data);
 VALUE rb_mutex_owned_p(VALUE self);
 
-VALUE rb_thread_scheduler_get(VALUE);
-VALUE rb_thread_scheduler_set(VALUE, VALUE);
+VALUE rb_thread_scheduler_get(VALUE thread);
+VALUE rb_thread_scheduler_set(VALUE thread, VALUE scheduler);
+
 VALUE rb_thread_scheduler_if_nonblocking(VALUE thread);
+VALUE rb_thread_current_scheduler();
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* Temporary.  This API will be removed (renamed). */

thread.c

@@ -3749,6 +3749,12 @@ rb_thread_scheduler(VALUE klass)
     return rb_thread_scheduler_if_nonblocking(rb_thread_current());
 }
 
+static VALUE
+rb_thread_current_scheduler()
+{
+    return rb_thread_scheduler_if_nonblocking(rb_thread_current());
+}
+
 VALUE
 rb_thread_scheduler_if_nonblocking(VALUE thread)
 {