Add getline_until to the io module.
Remove deprecated and useless io::flags.

The functionality has been streamlined into the format string of
io::fprint and friends.
Remove static_asserts from the algorithms to allow uintx to be used.

If the type passed isn't an integral type, then it'll still be
caught at compile time so the repercussion of removing it isn't too
bad.
Add mathematical constants.
Add is_prime function to test primality.

Fixed a bug with the Sieve of Eratosthenes.