Add Hash derive

[fredszaq]

Synopsis

This PR adds a new derive for the Hash trait. This does mostly the same as the derive bundled with rust std but with a few niceties :

• no overcontrain of generic types
• possibility to ignore fields with #[hash(skip)] (also honors #[eq(skip)] and #[partial_eq(skip)] to ensure trait expectations)
• possibility to use custom hash function on fields with #[hash(with(my_custom_hash_function)]

Solution

I used the PartialEq derive as an example to implement this, didn't have to do exotic things to get things to work

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)