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/rtl/mod.rs

@@ -1,5 +1,6 @@
 pub mod core;
 pub mod datetime;
+pub mod type_registry;
 
 use crate::ast::environment::Environment;