No longer let the GC replace dead keys with the LJ_TDEADKEY tag.

Important: this changes the semantics of the write barrier!
Carefully read the big comment block in lj_obj.h
This helps HREFK key slot specialization and allows safely hoisting
HREF/HREFK across GC steps, too (fix for a barely reproducible bug).
Dead keys are only removed during a table resize (as before).

src/lib_base.c

@@ -54,7 +54,7 @@ LJLIB_PUSH("upval")
 LJLIB_PUSH("thread")
 LJLIB_PUSH("proto")
 LJLIB_PUSH("function")
-LJLIB_PUSH("deadkey")
+LJLIB_PUSH("")  /* Unused. */
 LJLIB_PUSH("table")
 LJLIB_PUSH(top-8)  /* userdata */
 LJLIB_PUSH("number")