Accessors for root_task, current_task, safe_restore in TLS

In code which may be compiled against one Julia version but then gets
loaded in another (e.g. due to an update), it is problematic to directly
access members of jl_ptls_t, as this structure frequently changes
between Julia versions

The accessor function `jl_get_current_task` and `jl_get_root_task` avoid
this.

Note that macros `jl_current_task` and `jl_root_task` exist, but since
those are compiled into the code which includes `julia.h`, they do not
deal with the situation described above.

No alternatives exist for `jl_get_safe_restore` and
`jl_set_safe_restore`.

src/julia.h

@@ -2044,6 +2044,12 @@ typedef struct {
 #define jl_current_task (jl_get_ptls_states()->current_task)
 #define jl_root_task (jl_get_ptls_states()->root_task)
 
+JL_DLLEXPORT jl_value_t *jl_get_current_task(void);
+JL_DLLEXPORT jl_value_t *jl_get_root_task(void);
+
+JL_DLLEXPORT jl_jmp_buf *jl_get_safe_restore(void);
+JL_DLLEXPORT void jl_set_safe_restore(jl_jmp_buf *);
+
 // codegen interface ----------------------------------------------------------
 
 typedef struct {

src/julia_internal.h

@@ -1018,7 +1018,6 @@ JL_DLLEXPORT jl_value_t *(jl_array_data_owner)(jl_array_t *a);
 JL_DLLEXPORT int jl_array_isassigned(jl_array_t *a, size_t i);
 
 JL_DLLEXPORT uintptr_t jl_object_id_(jl_value_t *tv, jl_value_t *v) JL_NOTSAFEPOINT;
-JL_DLLEXPORT jl_value_t *jl_get_current_task(void);
 JL_DLLEXPORT void jl_set_next_task(jl_task_t *task);
 
 // -- synchronization utilities -- //

src/task.c

@@ -609,16 +609,28 @@ JL_DLLEXPORT jl_value_t *jl_get_current_task(void)
     return (jl_value_t*)ptls->current_task;
 }
 
-#ifdef JL_HAVE_ASYNCIFY
-JL_DLLEXPORT jl_ucontext_t *task_ctx_ptr(jl_task_t *t)
+JL_DLLEXPORT jl_value_t *jl_get_root_task(void)
 {
-    return &t->ctx;
+    jl_ptls_t ptls = jl_get_ptls_states();
+    return (jl_value_t*)ptls->root_task;
 }
 
-JL_DLLEXPORT jl_value_t *jl_get_root_task(void)
+JL_DLLEXPORT jl_jmp_buf *jl_get_safe_restore(void)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    return (jl_value_t*)ptls->root_task;
+    return (jl_value_t*)ptls->safe_restore;
+}
+
+JL_DLLEXPORT void jl_set_safe_restore(jl_jmp_buf *sr)
+{
+    jl_ptls_t ptls = jl_get_ptls_states();
+    ptls->safe_restore = sr;
+}
+
+#ifdef JL_HAVE_ASYNCIFY
+JL_DLLEXPORT jl_ucontext_t *task_ctx_ptr(jl_task_t *t)
+{
+    return &t->ctx;
 }
 
 void JL_DLLEXPORT jl_task_wait()