Implement the fact a BigInt is a VirtualString.

* There seems to be no smart way as of Boost <= 1.54.0
  to know the length of a BigInt in its base 10 representation.
  Using log() is likely slow. msb() is introduced in Boost 1.55
  (but it is too recent). Anyway, since we will most likely do the
  actual conversion to a string, perf is at most twice slower
  than it could be (we call twice str() instead of once + length computation).