github.com/shabbyrobe/go-num (go): Package num provides uint128 (U128) and int128 (I128) 128-bit integer types, covering most of the big.Int API.  `I128` is a signed "two's complement" implementation that should behave the same way on overflow as `int64`.  `U128` and `I128` are immutable value types by default; operations always return a new value rather than mutating the existing one.  Performance on x86-64/amd64 architectures is the focus.  Simple example:  64-bit helpers:  U128 and I128 can be created from a variety of sources:  U128 and I128 support the following formatting and marshalling interfaces:
Keywords: bigint|biginteger|bignum|go|golang|int128|number|uint128