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/bin/scryer-prolog.rs

@@ -0,0 +1,11 @@
+fn main() {
+    use nix::sys::signal;
+    use scryer_prolog::read::readline;
+    use scryer_prolog::*;
+
+    let handler = signal::SigHandler::Handler(handle_sigint);
+    unsafe { signal::signal(signal::Signal::SIGINT, handler) }.unwrap();
+
+    let mut wam = machine::Machine::new(readline::input_stream(), machine::Stream::stdout());
+    wam.run_top_level();
+}