Owners: propose stack_array and dyn_array

[MatthewBeshay]

Summary
Propose adding two Owners listed in the README as not yet implemented: stack_array and dyn_array. Goal is a minimal, idiomatic implementation that interoperates with gsl::span and follows the C++ Core Guidelines intent.

Motivation
The README’s Supported features - Owners section lists:

• stack_array - a stack-allocated array
• dyn_array - a heap-allocated array
  This issue asks for confirmation that the project would like these implemented here, and for guidance on a minimal surface area.

Proposed scope (sketch)

• gsl::stack_array<T, N>
  • Fixed-size owner; provides data(), size(), begin(), end(), operator[], at().
  • Interop with span via gsl::span{arr} and/or a view() helper.
• gsl::dyn_array
  • RAII owner for T[] with data(), size(), begin(), end(), operator[], at().
  • Movable; not resizable to keep scope small.
  • Interop with span as above.

[carsonRadtke]

Thanks for reporting this issue! Microsoft/GSL serves as an implementation of the specifications provided by the C++ Core Guidelines (isocpp/CppCoreGuidelines). Any proposed specification changes should be initiated in the guidelines repo and the necessary actions will be taken here.

That being said, there is already a proposal for gsl::dyn_array.

As for gsl::stack_array, it seems like what you're proposing has size known at runtime (basically a std::array with bounds checking). This is not what the C++ Core Guidelines specifies. The guidelines require a type that is gsl::stack_array<T> and size (which is fixed) is not specified until runtime. Unfortunately, the compiler does not currently provide the facilities necessary to implement such a type.