Fixed symbols so that interned symbols are only weakly referenced.

Fixed symbols so that interned symbols are only weakly referenced.

* libguile/symbols.c (lookup_interned_symbol): When a deleted weak pair
  is encountered, remove it and update the hashtable's item count.  Also,
  check the hash table threshold and trigger a rehashing if needed.
  (scm_i_c_mem2symbol): Allocate CELL using `scm_weak_car_pair ()' rather
  than `scm_cons ()'.
  (scm_i_mem2symbol): Likewise.
  (scm_symbols_prehistory): Don't invoke `scm_permanent_object ()' for
  SYMBOLS.

git-archimport-id: lcourtes@laas.fr--2005-libre/guile-core--boehm-gc--1.9--patch-44