Make the beancount parser fully reentrant

[Daniele Nicolodi]

Hello Martin,

during the weekend I went on a yak shaving expedition and I hacked the
beancount flex and bison parser to be fully reentrant eliminating all
global state. I've done it mostly as an exercise to see how the code
would like but from your comments in the code I had the impression that
this could be something that you may be interested into (and probably
was not done when the code was origianally written because it requires
some fighting with flex and bison).

I am fairly pleased with how the thing turned out but it would requires
a bit of polishing to be ready to send a patch upstream. Before I invest
some time in the less fun details, I would like to know: are you
interested in reviewing such a patch?

Thank you!

Best,
Daniele