feat: introduce std::span for c++20 dependencies

[k-wasniowski]

Use std::span when available, fall back to gsl_lite::span

Summary

Introduces a compile-time selection between std::span (C++20) and gsl_lite::span for the span type used across sframe's public API. Consumers building against a standard library that provides std::span no longer pull in <gsl-lite/gsl-lite.hpp> transitively through sframe headers.

Motivation

Downstream integrators on modern C++ toolchains (e.g. WebRTC on C++20) do not want gsl-lite in their include graph. Since C++20's std::span is a drop-in replacement for the subset of gsl::span that sframe uses, we can select it automatically and keep gsl-lite only as a C++17 fallback.

Changes

• New header include/sframe/span.h that:
  • Includes <version> when available to make feature-test macros visible.
  • When __cpp_lib_span >= 202002L is defined, includes <span> and exposes SFRAME_NAMESPACE::span as an alias for std::span.
  • Otherwise includes <gsl-lite/gsl-lite.hpp> and aliases SFRAME_NAMESPACE::span to gsl_lite::span.
• include/sframe/sframe.h and include/sframe/vector.h:
  • Drop the direct <gsl-lite/gsl-lite.hpp> include and the namespace gsl = ::gsl_lite; alias.
  • Include <sframe/span.h> and use the unqualified span alias (input_bytes / output_bytes unchanged in signature).
  • vector's span conversion operators construct the span from (pointer, size) so the same code compiles under either backend without relying on CTAD deduction differences.

[Copilot]

🟡 Changes recommended

The span backend is selected per translation unit, which can cause ABI/link mismatches between the compiled library and consumers built under different C++ standards.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR introduces a new SFRAME_NAMESPACE::span abstraction that uses std::span on C++20-capable standard libraries and falls back to gsl_lite::span otherwise, aiming to avoid pulling gsl-lite transitively through public headers on modern toolchains.

Changes:

• Added include/sframe/span.h to select between std::span and gsl_lite::span at compile time.
• Updated public API aliases in include/sframe/sframe.h to use the new span abstraction (input_bytes / output_bytes).
• Updated include/sframe/vector.h to accept/convert via the new span type and to construct spans from (pointer, size).

File summaries

File	Description
include/sframe/span.h	New compatibility header that selects std::span vs gsl_lite::span.
include/sframe/sframe.h	Switches input_bytes / output_bytes to the new span alias and removes direct gsl-lite exposure.
include/sframe/vector.h	Migrates constructors/append/conversions from gsl::span to span.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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[Copilot]

🟡 Changes recommended

The current span.h feature detection checks __cpp_lib_span before including <span>, which can incorrectly select the gsl-lite fallback even when std::span is available.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The new feature-detection header can select std::span in non-C++20 language modes based solely on __cpp_lib_span, which can break intended C++17 fallback behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

CI and build configuration should validate/support the C++20 std::span path and avoid overriding integrators’ chosen C++ standard when used as a subproject.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite