Implement TraceIfFalse type-checking and AST transformation.

This caused me some trouble. In my first approach, I ended up having
  multiple traces because nested values would be evaluated twice; once
  as condition, and once as part of the continuation.

  To prevent this, we can simply evaluate the condition once, and return
  plain True / False boolean as outcome. So this effectively transforms any
  expression:

  ```
  expr
  ```

  as

  ```
  if expr { True } else { trace("...", False) }
  ```

crates/aiken-lang/src/tests/check.rs

@@ -1,5 +1,5 @@
 use crate::{
-    ast::{ModuleKind, TypedModule, UntypedModule},
+    ast::{ModuleKind, Tracing, TypedModule, UntypedModule},
     builtins, parser,
     tipo::error::{Error, Warning},
     IdGenerator,
@@ -24,7 +24,14 @@ fn check_module(
     module_types.insert("aiken".to_string(), builtins::prelude(&id_gen));
     module_types.insert("aiken/builtin".to_string(), builtins::plutus(&id_gen));
 
-    let result = ast.infer(&id_gen, kind, "test/project", &module_types, &mut warnings);
+    let result = ast.infer(
+        &id_gen,
+        kind,
+        "test/project",
+        &module_types,
+        Tracing::KeepTraces,
+        &mut warnings,
+    );
 
     result
         .map(|o| (warnings.clone(), o))