LayoutMapping requirements should constrain `is_always_*()` to be `constexpr`

[dhollman]

This is because the primary use case is for constraining algorithms. For instance, the following should be prohibited:

extern bool foo;
struct _my_mapping {
  /* ... */
  static bool is_always_contiguous() { return foo; }
};

I think the way we should do this is to say in the table that the expression that needs to be valid is std::bool_constant<M::is_always_contiguous()>{}

[crtrott]

Agreed.