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/allocator.rs

@@ -9,7 +9,7 @@ use crate::targets::*;
 use std::cell::Cell;
 use std::rc::Rc;
 
-pub trait Allocator<'a> {
+pub(crate) trait Allocator<'a> {
     fn new() -> Self;
 
     fn mark_anon_var<Target>(&mut self, _: Level, _: GenContext, _: &mut Vec<Target>)