Remove Str and Slice swaps

[dtolnay]

Both of these types are "is_trivially_move_constructible" and "is_trivially_move_assignable", so the default std::swap would already correctly do exactly the same thing that these are doing.

We keep the custom swap on Box, String, Vec which are not trivially movable, their moves do stuff to the source object to move around ownership.

FYI @capickett

[capickett]

A small note here is that this changes means that

rust::Str s1 = "abc";
rust::Str s2 = "def";
s1.swap(s2);

Will now no longer compile. Referencing the STL, it appears std::string_view exposes a member swap https://en.cppreference.com/w/cpp/string/basic_string_view/swap. Perhaps worth keeping the member and implementing as:

void Str::swap(Str &rhs) noexcept {
  std::swap(*this, rhs);
}

[dtolnay]

Thanks! Good catch, I was only paying attention to the "std::swap"-style usage. Thanks for the link, it seems clear that we should have the member functions to match string_view. I've put those back in #646.