Replace memset with fill to work for non-trivial scalars.

For custom scalars, zero is not necessarily represented by
a zeroed-out memory block (e.g. gnu MPFR). We therefore
cannot rely on `memset` if we want to fill a matrix or tensor
with zeroes. Instead, we should rely on `fill`, which for trivial
types does end up getting converted to a `memset` under-the-hood
(at least with gcc/clang).

Requires adding a `fill(begin, end, v)` to `TensorDevice`.

Replaced all potentially bad instances of memset with fill.

Fixes #2245.

unsupported/test/cxx11_tensor_assign.cpp

@@ -25,10 +25,8 @@ static void test_1d()
   vec1(4) = 23; vec2(4) = 4;
   vec1(5) = 42; vec2(5) = 5;
 
-  int col_major[6];
-  int row_major[6];
-  memset(col_major, 0, 6*sizeof(int));
-  memset(row_major, 0, 6*sizeof(int));
+  int col_major[6] = {0};
+  int row_major[6] = {0};
   TensorMap<Tensor<int, 1> > vec3(col_major, 6);
   TensorMap<Tensor<int, 1, RowMajor> > vec4(row_major, 6);
 
@@ -88,10 +86,8 @@ static void test_2d()
   mat2(1,1) = 4;
   mat2(1,2) = 5;
 
-  int col_major[6];
-  int row_major[6];
-  memset(col_major, 0, 6*sizeof(int));
-  memset(row_major, 0, 6*sizeof(int));
+  int col_major[6] = {0};
+  int row_major[6] = {0};
   TensorMap<Tensor<int, 2> > mat3(row_major, 2, 3);
   TensorMap<Tensor<int, 2, RowMajor> > mat4(col_major, 2, 3);
 
@@ -148,10 +144,8 @@ static void test_3d()
     }
   }
 
-  int col_major[2*3*7];
-  int row_major[2*3*7];
-  memset(col_major, 0, 2*3*7*sizeof(int));
-  memset(row_major, 0, 2*3*7*sizeof(int));
+  int col_major[2*3*7] = {0};
+  int row_major[2*3*7] = {0};
   TensorMap<Tensor<int, 3> > mat3(col_major, 2, 3, 7);
   TensorMap<Tensor<int, 3, RowMajor> > mat4(row_major, 2, 3, 7);