st.c (new_size): use next_pow2 function

Reduces object size slightly on x86-64:

   text	   data	    bss	    dec	    hex	filename
2782359	  22400	  71880	2876639	 2be4df	ruby.orig
2781831	  22400	  71880	2876111	 2be2cf	ruby.pow2

And on 32-bit x86:

   text	   data	    bss	    dec	    hex	filename
2814751	  12100	  30552	2857403	 2b99bb	ruby.orig
2814051	  12100	  30552	2856703	 2b96ff	ruby.pow2

This is not a performance-critical function, but the
smaller icache footprint seems worth it.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47767 b2dd03c8-39d4-4d8f-98ff-823fe69b080e