Allow converting Big* to U/Int128

[BlobCodes]

Feature Request

It would be useful to allow converting a BigInt (and other Big objects) into a U/Int128.
This is already possible for all other primitive integers:

BigFloat.new("1").to_u64 #=> 1
BigFloat.new("1").to_u128 #=> undefined method 'to_u128' for BigFloat
BigInt.new("1").to_i128! #=> undefined method 'to_i128!' for BigInt