Add "value_type" to vec

Add a new type alias `value_type` to `vec`, making it more consistent
with `marray`.  This makes it much easier to define the templated
versions of the builtin functions because we can rely on this type
alias for both `vec` and `marray`.

adoc/headers/vec.h

@@ -35,6 +35,7 @@ struct elem {
 template <typename DataT, int NumElements> class vec {
  public:
   using element_type = DataT;
+  using value_type = DataT;
 
 #ifdef __SYCL_DEVICE_ONLY__
   using vector_t = __unspecified__;