Consolidate simple component headers (large, background_thread, bin)

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

- large_externs.h -> large.h: renamed; it was a single-purpose
  function-prototype file.
- background_thread_structs.h + background_thread_externs.h ->
  background_thread.h: merged.  background_thread_inlines.h is kept
  separate because it depends on arena_inlines_a.h.
- bin_inlines.h folded into bin.h, along with BIN_SHARDS_MAX /
  N_BIN_SHARDS_DEFAULT from bin_types.h.  bin.h carries a forward decl
  of arena_binind_div_info (declared in arena_externs.h) so it stays
  hermetic without re-introducing the bin.h <-> arena_externs.h cycle.
- tsd_binshards.h (new): houses tsd_binshards_t and its zero
  initializer.  Keeping these out of bin.h lets tsd_internals.h pull in
  just what it needs during X-macro expansion, avoiding bin.h's mutex.h
  dependency (mutex.h itself depends on TSD machinery, so routing it
  through tsd_internals.h forms a chicken-and-egg).

jemalloc_internal_includes.h: drops the now-redundant references to
the deleted/merged headers.

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) {