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/CMakeLists.txt b/CMakeLists.txt
index 3e6b40d..2c3c59d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,6 +18,7 @@
 
 include(pw_build/pigweed.cmake)
 
+add_subdirectory(pw_assert)
 add_subdirectory(pw_base64)
 add_subdirectory(pw_checksum)
 add_subdirectory(pw_cpu_exception)