lib/libhydrogen: Add new libhydrogen submodule.

This library is a small and easy-to-use cryptographic library which is well
suited to embedded systems.
Signed-off-by: Damien George <damien@micropython.org>

.gitmodules

@@ -36,3 +36,6 @@
 [submodule "lib/nxp_driver"]
 	path = lib/nxp_driver
 	url = https://github.com/hathach/nxp_driver.git
+[submodule "lib/libhydrogen"]
+	path = lib/libhydrogen
+	url = https://github.com/jedisct1/libhydrogen.git

libhydrogen @ 5c5d5130

+Subproject commit 5c5d513093075f7245ea522101b17c50aa579af2