Minor size reduction on 32-bit x86:

   text	   data	    bss	    dec	    hex	filename
  13742	     24	      0	  13766	   35c6	st.o
  14166	     24	      0	  14190	   376e	st-orig.o

Public API change to be proposed separately.

* st.c (find_entry): constify st_table*
  (find_packed_index_from): ditto
  (find_packed_index): ditto
  (get_keys): ditto
  (get_values): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51438 b2dd03c8-39d4-4d8f-98ff-823fe69b080e