Make tracing configurable, when relevant.

Tracing is now turn OFF by default when:

  - building project
  - building documentation
  - building dependencies

  It can be turned ON only when building project using `--keep-traces`.
  That means it's not possible to build dependencies with traces. The
  address `--rebuild` flag will also rebuild without traces.

  Tracing is however turn ON by default when:

  - checking the project (and running tests).

  In this scenario, tracing can be disabled using `--no-traces` (if for
  example, one want to analyze the execution units of specific functions
  without having to manually remove traces from code).

crates/aiken-lang/src/ast.rs

@@ -1023,6 +1023,16 @@ pub enum Tracing {
     KeepTraces,
 }
 
+impl From<bool> for Tracing {
+    fn from(keep: bool) -> Self {
+        if keep {
+            Tracing::KeepTraces
+        } else {
+            Tracing::NoTraces
+        }
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq)]
 pub struct Span {
     pub start: usize,