Various Improvements

- removed many panics from the pipeline
- extracted project and const tree to representations
- extended STL list support
- improved loops

src/pipeline/mod.rs

@@ -8,7 +8,3 @@ mod project_tree;
 mod source_loader;
 
 pub use parse_layer::parse_layer;
-pub use project_tree::{
-  collect_consts, collect_rules, from_const_tree, vname_to_sym_tree, ConstTree,
-  ProjectExt, ProjectModule, ProjectTree,
-};