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.

test/unit/bin.c

@@ -1,5 +1,8 @@
 #include "test/jemalloc_test.h"
 
+extern void bin_dalloc_locked_begin(
+    bin_dalloc_locked_info_t *info, szind_t binind);
+
 #define INVALID_ARENA_IND ((1U << MALLOCX_ARENA_BITS) - 1)
 
 /* Create a page-aligned mock slab with all regions free. */

test/unit/slab.c

@@ -1,5 +1,8 @@
 #include "test/jemalloc_test.h"
 
+extern void bin_dalloc_locked_begin(
+    bin_dalloc_locked_info_t *info, szind_t binind);
+
 #define INVALID_ARENA_IND ((1U << MALLOCX_ARENA_BITS) - 1)
 
 TEST_BEGIN(test_bin_slab_regind) {