Include <cstdint> in one place, remove custom typedefs

Originating from
[this SO issue](https://stackoverflow.com/questions/65901014/how-to-solve-this-all-error-2-in-this-case),
some win32 compilers define `__int32` as a `long`, but MinGW defines
`std::int32_t` as an `int`, leading to a type conflict.

To avoid this, we remove the custom `typedef` definitions for win32.  The
Tensor module requires C++11 anyways, so we are guaranteed to have
included `<cstdint>` already in `Eigen/Core`.

Also re-arranged the headers to only include `<cstdint>` in one place to
avoid this type of error again.

unsupported/Eigen/CXX11/ThreadPool

@@ -33,7 +33,6 @@
 #if __cplusplus > 199711L || EIGEN_COMP_MSVC >= 1900
 #include <cstddef>
 #include <cstring>
-#include <stdint.h>
 #include <time.h>
 
 #include <vector>