Containers: implement also StringView and char concatenation

[mosra]

Because why do the extra work with getting a size of the view if we don't need to.

This however results in an ambiguity with ArrayView<[const ]char> + int, i.e., when both sides of the expression need an implicit conversion -- then a candidate is either the builtin operator+(const char*, std::ptrdiff_t) , or operator+(StringView, char). Possible solutions:

• Disable implicit ArrayView to pointer conversion, forcing people to do view.data() + 3 instead of view + 3? Needs to be done in a backwards-compatible way however, as a lot of code relies on this... so it's not an immediate solution.
  • Could fix also many other annoyances (like nasty ambiguity when creating a StringView from an ArrayView<char>, which also picks the const char* overload)
• Disable implicit conversion between ArrayView and StringView? Would make a lot of new APIs nasty (such as Path::write() no longer accepting string views for data), OTOH could also prevent errors when write() accidentally gets data first and filename last. A lot of code also relies on this already, meaning it needs backwards compat and thus this is also not an immediate solution.
• (Ew.) Move the operator+() to String.h so we can have it templated and add a nasty SFINAE that picks only directly a StringView. Or a String? Or .... ugh. This would also mean trying to call operator+() when only StringView.h is included would fail with "no such operation possible", which isn't really nice to users.