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.
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.
