Add std::string_view constructor to hashed_string

[skhaz]

Summary

Adds a convenience constructor accepting std::string_view directly, eliminating the need for manual .data() and .size() calls.

Motivation

As noted in PR #824, std::string_view support was mentioned as desirable but not included at the time. This PR addresses that gap.

Many codebases use std::string_view extensively, and users currently write verbose code:

// Before: verbose
const auto key = entt::hashed_string{sv.data(), sv.size()};

// After: clean and intuitive
const auto key = entt::hashed_string{sv};

Implementation

constexpr basic_hashed_string(std::string_view sv) noexcept
    : basic_hashed_string{sv.data(), sv.size()} {}

Zero overhead: Single conversion to string_view, then direct delegation.

SFINAE constraint: Excludes C-string pointers to avoid ambiguity with const_wrapper used by UDL operators (_hs, _hws).

Test Coverage

• Null-terminated string_view
• Sized string_view (prefixes)
• Substring in middle of buffer
• Empty string_view
• Single character
• string_view with embedded nulls
• All 184 existing tests pass

Checklist

• Compiles without warnings (C++17)
• All 184 core tests pass
• Zero overhead implementation
• Backwards compatible
• Comprehensive test coverage

Related

• PR Store size information in hashed string  #824: Store size information in hashed string
• Issue Support using entt::type_info as key in associative containers #658: Uses view.data(), view.size() pattern

/cc @skypjack

[skhaz]

Any chance to get a review on this?