Fix poke expect neg ints (#66)

* Implicit Int to BigInt fails sometimes when expect is passed an Int
Trying to create simple example has proven intractable
For expect, and poke methods that take BigInts I have created overloaded versions
that take Ints and explicitly convert to BigInt

* Implicit Int to BigInt fails sometimes when expect is passed an Int
remove implicit conversions from int to bigint, that forced negative int into positive value representation

* Per Donggyu, preserve (but rename) his Int => BigInt and Long => BigInt, and make them not implicit