perf(html5): use a hashmap to detect duplicate attributes

If there are more than 16 attributes, shift from doing strcmp to using
a hashmap for duplicate detection. The number 16 was chosen based on
the benchmark in #2568

I've introduced @tidwall's hashmap.c (MIT licensed and the copyright
appropriately copied in the LICENSE-DEPENDENCIES file) to have
something self-contained within the libgumbo codebase, rather than
using libxml2's xmlHash or ruby's st.c.

LICENSE-DEPENDENCIES.md

@@ -15,6 +15,7 @@ Note that this document is broken into multiple sections, each of which describe
   * [Native WindowsⓇ platform releases ("x64-mingw-ucrt")](#native-windows%E2%93%A1-platform-releases-x64-mingw-ucrt)
   * [JavaⓇ (JRuby) platform release ("java")](#java%E2%93%A1-jruby-platform-release-java)
 - [Appendix: Dependencies' License Texts](#appendix-dependencies-license-texts)
+  * [hashmap.c](#hashmapc)
   * [libgumbo](#libgumbo)
   * [libxml2](#libxml2)
   * [libxslt](#libxslt)
@@ -112,6 +113,34 @@ This section contains a subsection for each potentially-distributed dependency,
 Please see previous sections to understand which of these potential dependencies is actually distributed in the gem file you're downloading and using.
 
 
+### hashmap.c
+
+MIT
+
+https://github.com/tidwall/hashmap.c/blob/master/LICENSE
+
+    The MIT License (MIT)
+
+    Copyright (c) 2020 Joshua J Baker
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy of
+    this software and associated documentation files (the "Software"), to deal in
+    the Software without restriction, including without limitation the rights to
+    use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+    the Software, and to permit persons to whom the Software is furnished to do so,
+    subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in all
+    copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+    FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+    COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+    IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+    CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+
 ### libgumbo
 
 Apache 2.0

gumbo-parser/src/Makefile

@@ -13,9 +13,11 @@ gumbo_objs := \
 	char_ref.o \
 	error.o \
 	foreign_attrs.o \
+	hashmap.o \
 	parser.o \
 	string_buffer.o \
 	string_piece.o \
+	string_set.o \
 	svg_attrs.o \
 	svg_tags.o \
 	tag.o \