Convert between BigInts and BigUints
Convert between BigInts, BigUints, ints, and uints

Previously, conversion to ints, uints, and BigUints clamped the value
within the range of that datatype. With this commit, conversion
overflows fail the task. To handle overflows gracefully, use the new
to_*_opt() methods.
auto merge of #9014 : dcrewi/rust/convert-between-bigints, r=anasazi