AK+LibUnicode: Implement String::equals_ignoring_case without allocating

We currently fully casefold the left- and right-hand sides to compare
two strings with case-insensitivity. Now, we casefold one code point at
a time, storing the result in a view for comparison, until we exhaust
both strings.

Userland/Libraries/LibUnicode/String.cpp

@@ -6,6 +6,8 @@
 
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
+#include <AK/Utf32View.h>
+#include <AK/Utf8View.h>
 #include <LibUnicode/UnicodeUtils.h>
 
 // This file contains definitions of AK::String methods which require UCD data.
@@ -40,12 +42,60 @@ ErrorOr<String> String::to_casefold() const
     return builder.to_string();
 }
 
+class CasefoldStringComparator {
+public:
+    explicit CasefoldStringComparator(Utf8View string)
+        : m_string(string)
+        , m_it(m_string.begin())
+    {
+    }
+
+    bool has_more_data() const
+    {
+        return !m_casefolded_code_points.is_empty() || (m_it != m_string.end());
+    }
+
+    u32 next_code_point()
+    {
+        VERIFY(has_more_data());
+
+        if (m_casefolded_code_points.is_empty()) {
+            m_current_code_point = *m_it;
+            ++m_it;
+
+            m_casefolded_code_points = Unicode::Detail::casefold_code_point(m_current_code_point);
+            VERIFY(!m_casefolded_code_points.is_empty()); // Must at least contain the provided code point.
+        }
+
+        auto code_point = m_casefolded_code_points[0];
+        m_casefolded_code_points = m_casefolded_code_points.substring_view(1);
+
+        return code_point;
+    }
+
+private:
+    Utf8View m_string;
+    Utf8CodePointIterator m_it;
+
+    u32 m_current_code_point { 0 };
+    Utf32View m_casefolded_code_points;
+};
+
 // https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G34145
-ErrorOr<bool> String::equals_ignoring_case(String const& other) const
+bool String::equals_ignoring_case(String const& other) const
 {
     // A string X is a caseless match for a string Y if and only if:
     //     toCasefold(X) = toCasefold(Y)
-    return TRY(to_casefold()) == TRY(other.to_casefold());
+
+    CasefoldStringComparator lhs { code_points() };
+    CasefoldStringComparator rhs { other.code_points() };
+
+    while (lhs.has_more_data() && rhs.has_more_data()) {
+        if (lhs.next_code_point() != rhs.next_code_point())
+            return false;
+    }
+
+    return !lhs.has_more_data() && !rhs.has_more_data();
 }
 
 }