Improve `Let` statements lowering in MIR

Currently, the lowering of Let statements is not the cleanest, as translate_statement returns a single node but the statements can create multiple nodes in their bodies.

We should refactor with the following logic:

```rust
/// Translates a statement into a vector of operations. We return a vector as let statements contain all the statements in its scope.
fn translate_statement(&mut self, stmt: &'a ast::Statement) -> Result>, CompileError> {
match stmt {
ast::Statement::Let(let_stmt) => self.translate_let(let_stmt),
ast::Statement::Expr(expr) => Ok(vec![self.translate_expr(expr)?],
[...]
}
}

/// Translates a let statement, binding its value to its name for the scope of its body, and returning all the inner statements of its body.
fn translate_let(&mut self, let_stmt: &'a ast::Let) -> Result>, CompileError> {
[...]

// Translating all the statements of the let's body, and adding them to our returned vectors, flattened
let ret_value = let_stmt.body.iter()
.map(|stmt| self.translate_statement(stmt))
.collect::, _>>()?
.into_iter()
.flatten()
.collect();
Ok(ret_value)
}
```

Then, when we translate a statement, we handle all the inner statements like we do now. So if we are translating a fucntion / evaluator, we can directly insert all the statements into its body

_Originally posted by @Leo-Besancon in https://github.com/0xMiden/air-script/pull/449#discussion_r2313254144_