Also freeStackBuffer() and all the funky classic-C string management of a big
flat buffer divided into exactly 512 128-byte strings. Define StringVector as
a std::vector<std::string>, and use that instead.

Retain the behavior of clearing the vector if it exceeds 512 entries.

This eliminates the LLError::Log::flush(const std::ostringstream&, char*)
overload as well, with its baffling mix of std::string and classic-C (e.g.
strlen(out.str().c_str()).

If we absolutely MUST use a big memory pool for performance reasons, let's
use StringVector with allocators.