Allow tracepoints to use direct indexing for number->string translation

[David Howells]

Another feature that would be very nice to have is the ability to turn a
number into a string by direct array index rather than by table search or
serial ?: ternary operators.

This would make tracepoint viewing more efficient, though there is a potential
downside: if the value to be looked up is corrupt, it could index beyond the
end of the array.

David