pw_varint: Add varint module

This change adds a pw_varint module containing functions for encoding
and decoding variable-length integers.

Change-Id: I50bdf6d9d6762bffb93ee638683de53afed9c849
diff --git a/modules.gni b/modules.gni
index b0b6c6e..9c1d816 100644
--- a/modules.gni
+++ b/modules.gni
@@ -35,3 +35,4 @@
 dir_pw_string = "$dir_pigweed/pw_string"
 dir_pw_toolchain = "$dir_pigweed/pw_toolchain"
 dir_pw_unit_test = "$dir_pigweed/pw_unit_test"
+dir_pw_varint = "$dir_pigweed/pw_varint"