ffi: provide support for unions.

* ffi.c (union_s): New symbol variable.
(ffi_find_memb, ffi_memb_not_found): New static functions.
(ffi_union_in, ffi_union_put, ffi_union_get): New static
functions.
(make_ffi_type_union): New static function.
(ffi_struct_compile): Handle union syntax using
ffi_struct_compile to compile the member definitions to types,
and make_ffi_type_union to produce the type node.
(struct uni): New struct type.
(uni_struct, uni_struct_checked): New static functions.
(union_destroy_op, union_mark_op): New static functions.
(union_ops): New static struct.
(make_union_common, make_union_tft): New static functions.
(union_get_ptr, make_union, union_members, union_get,
union_put, union_in, union_out): New functions.
(ffi_init): Initialize union_s. Register intrinsics
make-union, union-members, union-get, union-put, union-in,
union-out.

* ffi.h (union_s, union_get_ptr, make_union, union_members,
union_get, union_put, union_in, union_out): Declared.

* txr.1: Documented unions.

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