split into lib and bin

* makes most pub things in src/ pub(crate) as not to expose things accidentally
  * only those things needed by src/bin/scryer-prolog.rs and tests/scryer.rs
    should be pub
* split src/main.rs into src/lib.rs and src/bin/scryer-prolog.rs
* add tests folder and run most of the files in src/tests with cargo test
  added bytes method to Stream in src/machine/streams.rs to check if stdout is as expected

src/tests/hello_world.pl

@@ -0,0 +1,6 @@
+:- use_module(library(debug)).
+:- use_module(library(format)).
+
+hello_world :- write('Hello World!'), nl.
+
+:- initialization(hello_world).