string.c (Init_frozen_strings): use st_init_table_with_size

All symbols have an fstring entry, and we initialize symbol tables
with 1000 bins.  This reduces resizes during startup.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47544 b2dd03c8-39d4-4d8f-98ff-823fe69b080e