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/targets.rs

@@ -5,7 +5,7 @@ use crate::forms::*;
 use crate::instructions::*;
 use crate::iterators::*;
 
-pub trait CompilationTarget<'a> {
+pub(crate) trait CompilationTarget<'a> {
     type Iterator: Iterator<Item = TermRef<'a>>;
 
     fn iter(_: &'a Term) -> Self::Iterator;