AK: Make CaseInsensitiveStringTraits allocation-free

Instead of calling String::to_lowercase(), do case-insensitive hashing
and comparison.

AK/String.h

@@ -305,8 +305,8 @@ struct Traits<String> : public GenericTraits<String> {
 };
 
 struct CaseInsensitiveStringTraits : public Traits<String> {
-    static unsigned hash(const String& s) { return s.impl() ? s.to_lowercase().impl()->hash() : 0; }
-    static bool equals(const String& a, const String& b) { return a.to_lowercase() == b.to_lowercase(); }
+    static unsigned hash(String const& s) { return s.impl() ? s.impl()->case_insensitive_hash() : 0; }
+    static bool equals(String const& a, String const& b) { return a.equals_ignoring_case(b); }
 };
 
 bool operator<(const char*, const String&);

AK/StringImpl.cpp

@@ -133,6 +133,11 @@ NonnullRefPtr<StringImpl> StringImpl::to_uppercase() const
     return const_cast<StringImpl&>(*this);
 }
 
+unsigned StringImpl::case_insensitive_hash() const
+{
+    return case_insensitive_string_hash(characters(), length());
+}
+
 void StringImpl::compute_hash() const
 {
     if (!length())

AK/StringImpl.h

@@ -75,6 +75,8 @@ public:
         return m_hash;
     }
 
+    unsigned case_insensitive_hash() const;
+
     bool is_fly() const { return m_fly; }
     void set_fly(Badge<FlyString>, bool fly) const { m_fly = fly; }