Make not_null<foo*> trivially copyable.

[rnlahaye]

This fixes an annoying -Wrange-loop-construct clang warning.

Done by creating a wrapper struct that uses the default move assignment operator for trivially move assignable pointers and the custom swap-based move assignment operator for others.

[pleroy]

Lint errors:

  | not_null.hpp:104 | runtime/explicit | High | 1
  | not_null.hpp:117 | runtime/explicit | High | 1
  | not_null.hpp:123 | build/include_what_you_use | Normal | 1
  | not_null.hpp:218 | whitespace/line_length | Low | 1
  | not_null.hpp:97 | whitespace/operators | Normal | 1

[eggrobin]

Change all of those to static_assert (cf. https://github.com/mockingbirdnest/Principia/blob/master/numerics/next_test.cpp which tests constexpr code).

[rnlahaye]

Done.

[pleroy]

The linter is still unhappy (and somewhat dumb):

not_null.hpp:124	build/include_what_you_use	Normal	1
Add #include <algorithm> for swap