Refactor testing guidelines and add specializer module

Update the testing section to clarify when warnings should be eliminated
and tests run.
Add the new `specializer` module to the compiler's public API.
Add the `type_registry` module to the `rtl` module's public API.

src/ast/compiler/mod.rs

@@ -5,6 +5,7 @@ pub mod upvalues;
 pub mod dumper;
 pub mod macros;
 pub mod type_checker;
+pub mod specializer;
 
 pub use binder::*;
 pub use bound_nodes::*;
@@ -13,3 +14,4 @@ pub use upvalues::*;
 pub use dumper::*;
 pub use macros::*;
 pub use type_checker::*;
+pub use specializer::*;