Replaced Libtock driver clock with embedded_time::Clock (#422)

* Replaced Libtock driver clock with embedded_time::Clock

* Add unittest and address some comments

* Add unittest and address some comments

Cargo.toml

@@ -21,6 +21,7 @@ arrayref = "0.3.6"
 subtle = { version = "2.2", default-features = false, features = ["nightly"] }
 # This import explicitly locks the version.
 serde_json = { version = "=1.0.69", default-features = false, features = ["alloc"] }
+embedded-time = "0.12.1"
 
 [features]
 debug_allocations = ["lang_items/debug_allocations"]