refactor: consolidate Dash-specific utils between C++23 polyfills (util/std23.h) and helpers (util/helpers.h)

[kwvg]

Motivation

The upgrade to C++20 (dash#6380) allowed us to drop most of our polyfills meant as bridge from C++17, though we still have utils beyond the ones inherited from upstream that have accumulated over time, most of them, seeking to bridge C++23 features while others are entirely custom or serve as convenience functions or thin wrappers.

Some of them include:

• dash#4622 for util/ranges.h (mostly now an alias to std::ranges after C++20 migration except for find_if_opt, which is custom)
• dash#4788 for util/irange.h (equivalent to std::views::iota, introduced in C++20)
• dash#5059 for util/enumerate.h (equivalent to std::views::enumerate, introduced in C++23)
• dash#5210 for util/underlying.h (equivalent to std::to_underlying, introduced in C++23)
• 0a4e726 for util/pointer.h (custom std::shared_ptr helper)

This pull requests compacts all of those helpers into two files:

• util/helpers.h (custom helpers, replacing util/irange.h, util/pointer.h, util/ranges.h)
• util/std23.h (polyfill for C++23, replacing util/enumerate.h and util/underlying.h)

And additionally, introduces additional capabilities utilised in Dash-specific code:

• std23::ranges::contains (equivalent to std::ranges::contains)
• std23::ranges::fold_left (equivalent to std::ranges::fold_left)
• util::to_string(bool) (equivalent to value ? "true" : "false")

Additional Information

• ToUnderlying() needed to be renamed to to_underlying() to match with the standard library naming so we can discontinue the polyfill easily when we migrate to C++23 by just going s/std23/std/g.

• While iranges::range is equivalent to std::views::iota, the latter requires both min and max to be of the same type but literal 0 has a type incompatible with size_t, requiring size_t{0} to compile. As propagation of these changes for each instance is bothersome, util::irange() exists as a thin wrapper that wraps the literal around the type for max

Breaking Changes

None expected.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation (note: N/A)
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)