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
do not include Math::Int64 as submodule + clean-up int64 support
Changes: update