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-project/src/options.rs

@@ -1,5 +1,8 @@
+use aiken_lang::ast::Tracing;
+
 pub struct Options {
     pub code_gen_mode: CodeGenMode,
+    pub tracing: Tracing,
 }
 
 pub enum CodeGenMode {