Ansgar fixed a bunch of problems in bigunsignedint - and improved the
corresponding test! yeah!

* p/ansgar/FS1644-bigunsignedint:
  Rework bigunsignedinttest.cc.
  bigunsignedint: Remove unneeded variable in operator%.
  bigunsignedint: Fix operator-.
  bigunsignedint: Throw an exception when constructing from a negative integer.
  bigunsignedint: make overload unambiguous for all integer types
  Use fixed-width integer types (C++11)