- public and private headers
- regtests
- check q and r signs
--
- to_string methods
- generic facade for bignums and rationals?
--
- use auto stuff in makefile
- privatize members
- fix quotient_remainder() (make static?)
- constify
- replace long with int32?
- #define BIGNUM_EXCEPTION abort
- de-inline

reg test multiply:
bignum_repr(a, 10) = 1208925819614629174706176
bignum_repr(b, 10) = 1208925819614629174706176
bignum_repr(c, 10) = 1461501637330902918203684832716283019655932542976

