style: clippy changes

[mrghosti3]

Changes:

• use String:new() instead of format!("")
• use is_empty() instead of s == ""
• use reference iterator instead of into_iter
• replace write! + '\n' with writeln!
• remove mut from vars with type &mut ...
• replace single branch match with if pattern match
• replace Stack::to_string with Display trait
• replace match with pattern matching in fn main
• use reference to file name, instead of direct ownership

[mrghosti3]

This contains similar changes to #70, but only sticks to clippy changes, leaving Stack in the code.