Fix SmallInt overflow condition in build(VM, int64IfDifferentFromNativeInt)
Use the builders system to build BigInts

* Restricts the impact to 3 methods in VirtualMachineEnvironment
  instead of 4 methods in VM and 4 methods in VMEnv.
Merge pull request #166 from eregon/bigint

Refactor the BigInt entry points to use standard builders