Refac unnecessary ref mut pattern matching

[marcospb19]

As an attempt to make the code more idiomatic.

[marcospb19]

Knowing "idiomatic Rust" is a subjective concept, feel free to close this if you don't see benefit in it.

I believe it is important to have the examples without it because I see ref mut as an anti-pattern when it is not necessary anymore.

[marcospb19]

I didn't include changes like this:

// From
if let Some(ref mut dac) = maybe_dac {
// To
if let Some(dac) = &mut maybe_dac {

That replace ref mut by &mut, I prefer the latter, but I guess in that case it doesn't matter A or B.