Consider mapping java.math.BigInteger with Bignum

We'd probably like to add something for this in 1.0, if not full-features coercion and mapping.

This got resovlved incidentally at some point, perhaps with various javasupport improvements over the past several months.

I find it confusing that this example doesn't work:

irb(main):003:0> java.math.BigInteger.new("2").pow(3).pow(4)
NoMethodError: undefined method `pow' for 8:Bignum or inaccessible method `pow' for 8:Bignum
from (irb):3:in `method_missing'
from (irb):3:in `binding'

pow() should return a BigInteger but instead returns a Bignum, which doesn't know the pow method.