Fix clippy warnings

[awidegreen]

I used cargo +nightly clippy to extract the warnings.

Most of the changes could be made by:

• using refs whenever possible
• change usage of unwrap_or to unwrap_or_else, see clippy: warning 'or_fun_call'
• change _.filter(_).next(_) to _.fínd(_): see clippy warning 'filter_next'
• change usage of const var: &'static str to const var: &str, see clippy warning 'const_static_lifetime'

[maralla]

Thanks!