Improve performance of http::string_to_verb

`operator==` performs better than the hand-written comparison routine.

fix #1636

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

include/boost/beast/core/impl/string.ipp

@@ -0,0 +1,71 @@
+//
+// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/boostorg/beast
+//
+
+#ifndef BOOST_BEAST_IMPL_STRING_IPP
+#define BOOST_BEAST_IMPL_STRING_IPP
+
+#include <boost/beast/core/string.hpp>
+
+#include <algorithm>
+
+namespace boost {
+namespace beast {
+
+bool
+iequals(
+    beast::string_view lhs,
+    beast::string_view rhs)
+{
+    auto n = lhs.size();
+    if(rhs.size() != n)
+        return false;
+    auto p1 = lhs.data();
+    auto p2 = rhs.data();
+    char a, b;
+    // fast loop
+    while(n--)
+    {
+        a = *p1++;
+        b = *p2++;
+        if(a != b)
+            goto slow;
+    }
+    return true;
+slow:
+    do
+    {
+        if(detail::ascii_tolower(a) != detail::ascii_tolower(b))
+            return false;
+        a = *p1++;
+        b = *p2++;
+    }
+    while(n--);
+    return true;
+}
+
+bool
+iless::operator()(
+        string_view lhs,
+        string_view rhs) const
+{
+    using std::begin;
+    using std::end;
+    return std::lexicographical_compare(
+        begin(lhs), end(lhs), begin(rhs), end(rhs),
+        [](char c1, char c2)
+        {
+            return detail::ascii_tolower(c1) < detail::ascii_tolower(c2);
+        }
+    );
+}
+
+} // beast
+} // boost
+
+#endif