BUILD: makefile: integrate the hpack tools

The few hpack development tools are now integrated into the main
makefile, which allows to remove the original one which was causing
lots of build warnings. A README was added to explain how to build
instead.
diff --git a/.github/workflows/contrib.yml b/.github/workflows/contrib.yml
index 4c328fd..d84c99d 100644
--- a/.github/workflows/contrib.yml
+++ b/.github/workflows/contrib.yml
@@ -21,4 +21,4 @@
         make dev/poll/poll
     - name: Compile dev/hpack
       run: |
-        make -C dev/hpack
+        make dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht