Consolidate simple component headers (large, background_thread, bin)

Fold historical *_types/_structs/_externs/_inlines splits where the
layering is no longer load-bearing.

- large_externs.h -> large.h: rename; it was a single-purpose
  function-prototype file.
- background_thread_structs.h + background_thread_externs.h ->
  background_thread.h: merge.  background_thread_inlines.h stays
  separate because it depends on arena_inlines_a.h.
- bin_types.h -> bin.h: BIN_SHARDS_MAX / N_BIN_SHARDS_DEFAULT join the
  bin_t struct and the bin_dalloc_locked_info_t type.  bin_inlines.h
  stays separate so TUs that only need the bin_t type don't pull in
  the tcache-flush inline closure.  The two bin_stats_* helpers move
  from bin.h into bin_inlines.h so all bin inlines live together.
- tsd_binshards.h (new): houses tsd_binshards_t and its zero
  initializer.  Lets tsd_internals.h pull it in for X-macro expansion
  without dragging mutex.h -- mutex.h depends on TSD machinery, which
  would form a cycle through bin.h.

jemalloc_internal_includes.h drops the references to the deleted/
merged headers.

src/arena.c

@@ -36,6 +36,14 @@ static pa_central_t arena_pa_central_global;
 
 div_info_t arena_binind_div_info[SC_NBINS];
 
+JET_EXTERN void
+bin_dalloc_locked_begin(
+    bin_dalloc_locked_info_t *info, szind_t binind) {
+	info->div_info = arena_binind_div_info[binind];
+	info->nregs = bin_infos[binind].nregs;
+	info->ndalloc = 0;
+}
+
 size_t opt_oversize_threshold = OVERSIZE_THRESHOLD_DEFAULT;
 size_t oversize_threshold = OVERSIZE_THRESHOLD_DEFAULT;