travis: do not explicitly install Devel::CheckLib (it's included in dzil listdeps)
CompileTest.pm: compile with debug symbols
06-struct.t: fix dumb invalid memory accesses
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
use Math::BigInt explicitly and do string comparison