Real.toString

[Henry Cejtin]

For Real.toString, the more important question is the thread safety.  On the
one hand, it really is going to cost us performance wise if we have to allocate
the buffer on every call.  On the other hand, the current code will clearly
fail in the most horrible of non-deterministic ways if we have fine-grained
enough context switching.  One way or another we have to decide which way
we are going.  I would argue that multiple threads is a bad idea (nothing
new from me).