feat: add std::string_view constructor to hashed_string

Adds a convenience constructor that accepts std::string_view directly,
eliminating the need to manually pass .data() and .size().

This addresses the gap mentioned in PR #824 where implicit conversion
to std::string_view was noted as desirable but not included.

Changes:
- Added basic_hashed_string(std::basic_string_view<value_type>) ctor
- Added corresponding test case StringViewConstructor

Example usage:
  std::string_view sv = "foobar";
  auto hs = entt::hashed_string{sv};  // Cleaner!

Related: PR #824

Author: skhaz

src/entt/core/hashed_string.hpp

@@ -3,6 +3,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <string_view>
 #include "fwd.hpp"
 
 namespace entt {
@@ -125,6 +126,13 @@ class basic_hashed_string: internal::basic_hashed_string<Char> {
         // NOLINTEND(cppcoreguidelines-pro-bounds-pointer-arithmetic)
     }
 
+    /**
+     * @brief Constructs a hashed string from a string view.
+     * @param sv String view to hash.
+     */
+    constexpr basic_hashed_string(std::basic_string_view<value_type> sv) noexcept
+        : basic_hashed_string{sv.data(), sv.size()} {}
+
     /**
      * @brief Constructs a hashed string from an array of const characters.
      * @tparam N Number of characters of the identifier.

test/entt/core/hashed_string.cpp

@@ -85,6 +85,17 @@ TEST_F(HashedString, Correctness) {
     ASSERT_EQ(entt::hashed_string{"foobar"}.size(), 6u);
 }
 
+TEST_F(HashedString, StringViewConstructor) {
+    const std::string_view view{"foobar"};
+    const std::string_view view_sized{"foobar__", 6};
+
+    ASSERT_EQ(entt::hashed_string{view}, expected);
+    ASSERT_EQ(entt::hashed_string{view_sized}, expected);
+    ASSERT_EQ(entt::hashed_string{view}.size(), 6u);
+    ASSERT_EQ(entt::hashed_string{view_sized}.size(), 6u);
+    ASSERT_STREQ(entt::hashed_string{view}.data(), "foobar");
+}
+
 TEST_F(HashedString, Order) {
     using namespace entt::literals;
     const entt::hashed_string lhs = "foo"_hs;