Replace read_objects assoc list with two hash tables.

Replace read_objects assoc list with two hash tables.

For larger input files with lots of shared data structures, an
association list is too slow.

* src/lread.c (read_objects_map, read_objects_completed): New
variables, replacing read_objects.
(readevalloop): Initialize them with hash tables before starting a
top-level read, if they're not already empty hash tables, and reset
them to Qnil afterwards if something was added to the hash tables.
(read_internal_start): Likewise.
(read1): Store first the placeholder and later the newly read object
into read_objects_map under the specified object number.  If the new
object can contain a reference to itself, store it in
read_objects_completed.
(substitute_objects_recurse): Check read_objects_completed instead of
read_objects for the known possibly-recursive objects.
(syms_of_lread): Update initializations.