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

build.rs

@@ -44,7 +44,7 @@ fn main() {
     libraries
         .write_all(
             b"ref_thread_local::ref_thread_local! {
-    pub static managed LIBRARIES: IndexMap<&'static str, &'static str> = {
+    pub(crate) static managed LIBRARIES: IndexMap<&'static str, &'static str> = {
         let mut m = IndexMap::new();\n",
         )
         .unwrap();