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/debray_allocator.rs

@@ -14,7 +14,7 @@ use std::collections::BTreeSet;
 use std::rc::Rc;
 
 #[derive(Debug)]
-pub struct DebrayAllocator {
+pub(crate) struct DebrayAllocator {
     bindings: IndexMap<Rc<Var>, VarData>,
     arg_c: usize,
     temp_lb: usize,