Add support for big integers

[andig]

Imlementation is already here andig@5c87905 but I can't PR without pushing the rest of the fork over. This is the final piece I'd need to deprecate my fork.

[gonzofy]

@andig question: that seems like a simple PR to create since the change is relatively small. What's hard about pulling it over? Is there something else about the implementation that the PR would require in order to work?