change mutable reference syntax to &'a mut T

[RaphiMuehlbacher]

What does this PR do?

Closes #426

How does it work, what questions do you have?

I changed the order of how the variant RefMut gets parsed by parsing the keyword mut after the lifetime (&'a mut T instead of &mut 'a T). Additionally I changed the debug representation of RefMut so that it matches the new syntax. Also updated all affected tests to reflect the new syntax.

AI disclosure

• I did not use any AI tools

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