Stmt: Introduce assert statement and related hooks

including two hooks called assertion_failure() and assertion_result() for
customization and tracking of assertion results.

testing/btest/Baseline/language.assert-misc/out

@@ -0,0 +1,13 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+f, lambda_<505728364269398358>
+{ 
+assert 0 < getpid(), fmt("my pid is funny: %s", getpid());
+}
+g, lambda_<8496146571423528161>
+{ 
+assert to_count("42") == 42;
+}
+test_function, test_function
+{ 
+assert 0 < getpid();
+}