bug #1382: move using std::size_t/ptrdiff_t to Eigen's namespace (still better than the global namespace!)

(grafted from b0db4eff367017f8b273b7d407b3fe0e86808ed2
)

Eigen/Core

@@ -321,12 +321,16 @@ inline static const char *SimdInstructionSetsInUse(void) {
 #error Eigen2-support is only available up to version 3.2. Please go to "http://eigen.tuxfamily.org/index.php?title=Eigen2" for further information
 #endif
 
+namespace Eigen {
+
 // we use size_t frequently and we'll never remember to prepend it with std:: everytime just to
 // ensure QNX/QCC support
 using std::size_t;
 // gcc 4.6.0 wants std:: for ptrdiff_t
 using std::ptrdiff_t;
 
+}
+
 /** \defgroup Core_Module Core module
   * This is the main module of Eigen providing dense matrix and vector support
   * (both fixed and dynamic size) with all the features corresponding to a BLAS library