Initial implementation of policy coverage (#146)

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/mod.rs

@@ -1,6 +1,9 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+#[cfg(feature = "coverage")]
+mod coverage;
+
 mod engine;
 mod lexer;
 mod parser;