add support for 64 bit ffi integer types on both 32bit and 64bit Perls

use Math::Int64 C API for marshalling/unmarshalling 64bit SV's
add Math::Int64 as submodule
add Math::Int64 build steps and dependencies
add Math::Int64 as explicit dependency in dist.ini
older perls do not support some bigint operator overloads

use Math::BigInt values explicitly in test comparisons