Improve stringbuffer_t performance

[pramsey]

Our borrowed stringbuffer_t implementation isn't very smart, speedwise, it needs some love and polish. For example avoid all this memory allocating and deallocating when printing formatted strings, print to a static buffer with snprintf and only do allocation tricks if you overrun the limit. Also, increase the buffer size more aggressively, allocate a buffer right at the start.

[pramsey]

Done as of r6359