Improve the experimental local and scoped allocation.

* configure.ac (HAVE_STRUCT_ATTRIBUTE_ALIGNED)
(HAVE_STATEMENT_EXPRESSIONS): New configure-time checks.
* src/alloc.c (local_string_init, local_vector_init):
New functions, defined if USE_LOCAL_ALLOCATORS.
Mostly, these are moved here from lisp.h, as it's not
clear it's worth making them inline.
* src/lisp.h (USE_STACK_LISP_OBJECTS): Default to false.
(GCALIGNED): Depend on HAVE_STRUCT_ATTRIBUTE_ALIGNED and
USE_STACK_LISP_OBJECTS, not on a laundry list.
(local_string_init, local_vector_init): New decls.
(union Aligned_Cons): New type.
(scoped_cons): Use it.  Give up on the char trick, as it's a too
much of a maintenance hassle; if someone wants this speedup
they'll just need to convince their compiler to align properly.
Conversely, use the speedup if struct Lisp_Cons happens to
be aligned even without a directive.  Better yet, help it along
by using union Aligned_Cons rather than struct Lisp_Cons.
(pointer_valid_for_lisp_object): Remove.  This check is not
necessary, since make_lisp_ptr is already doing it.  All uses removed.
(local_vector_init, local_string_init): Move to alloc.c.
(build_local_vector): Remove this awkward macro, replacing with ...
(make_local_vector): New macro, which acts more like a function.
Use statement expressions and use __COUNTER__ to avoid macro
capture.  Fall back on functions if these features are not supported.
(build_local_string, make_local_string): Likewise.

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