change foo-to-string formatting paths to use Fmt module

There are a few too many places where data structures are string-ified (Asm.expr64 or Common.size, for example) that should really be Fmt'ed (streaming pretty-printed). This is both more efficient and makes them more convenient to compose via the "%a" format-string type.

1. WONTFIX (not required for bootstrapping)

   Graydon Hoare at 2011-01-27 00:44:36