Base64.Decode has quadratic runtime on Go

[robin-aws]

See https://github.com/dafny-lang/dafny/blob/master/Source/DafnyStandardLibraries/examples/Base64Examples.dfy#L33.

This isn't surprising on JS since its runtime doesn't have the sequence concatenation optimization, but Go does. Encode doesn't have the same issue. I suspect that sequence slicing is at fault: it should be O(1) time in the Go backend but perhaps is unintentionally making a copy.