pw_assert: Initial assert module

This starts the assert and assert_basic modules, which are the beginning
of our assert foundation. Much more is needed; in particular the "tests"
don't really do much other than check for compilation.

Screenshot
==========

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  Welp, that didn't go as planned. It seems we crashed. Terribly sorry!

  CRASH MESSAGE

     Check failed: x (=50) > y (=51300): You SHOULD see this message

  CRASH FILE & LINE

     ../../pw_assert/assert_test.cc:46

  CRASH FUNCTION

     PigweedTestBody

==========

Change-Id: I9dbb460b3b33040d8009749f44b9fc174e2c8138
diff --git a/modules.gni b/modules.gni
index d543d55..9bf257b 100644
--- a/modules.gni
+++ b/modules.gni
@@ -16,6 +16,8 @@
 # allows modules to be moved or swapped out without breaking existing builds.
 # All module variables are prefixed with dir_.
 
+dir_pw_assert = "$dir_pigweed/pw_assert"
+dir_pw_assert_basic = "$dir_pigweed/pw_assert_basic"
 dir_pw_base64 = "$dir_pigweed/pw_base64"
 dir_pw_bloat = "$dir_pigweed/pw_bloat"
 dir_pw_boot_armv7m = "$dir_pigweed/pw_boot_armv7m"