Replace some calls to OutBuffer->printf with OutBuffer->writestring

Running the Windows version of DMD through a profiler shows vsprintf quite high up the call list (e.g. http://forum.dlang.org/post/ktgbft$m6i$1@digitalmars.com), and looking at the code a large chunk of those calls are from toDecoBuffer functions which just use a "%s" format string.

It should be more efficient to just call writestring instead - only makes a minor difference, but every little helps.