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.
bin_t is an arena implementation detail; tcache should not reach into
it. Extract the slab-address lookup into bin.c as bin_current_slab_addr,
and expose it to tcache only through arena_locality_hint(tsdn, arena,
szind), which composes bin_choose + bin_current_slab_addr.
