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/machine/term_stream.rs

@@ -78,7 +78,7 @@ impl<'a> TermStream for BootstrappingTermStream<'a> {
     }
 }
 
-pub struct LiveTermStream {
+pub(crate) struct LiveTermStream {
     pub(super) term_queue: VecDeque<Term>,
     pub(super) listing_src: ListingSource,
 }
@@ -93,7 +93,7 @@ impl LiveTermStream {
     }
 }
 
-pub struct LoadStatePayload {
+pub(crate) struct LoadStatePayload {
     pub(super) term_stream: LiveTermStream,
     pub(super) compilation_target: CompilationTarget,
     pub(super) retraction_info: RetractionInfo,