AK: Make StringView::hash() constexpr

This required moving string_hash() to its own header so that everyone
can see it.

AK/StringHash.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2018-2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace AK {
+
+constexpr u32 string_hash(char const* characters, size_t length)
+{
+    u32 hash = 0;
+    for (size_t i = 0; i < length; ++i) {
+        hash += (u32)characters[i];
+        hash += (hash << 10);
+        hash ^= (hash >> 6);
+    }
+    hash += hash << 3;
+    hash ^= hash >> 11;
+    hash += hash << 15;
+    return hash;
+}
+
+}
+
+using AK::string_hash;