Add support for fields

[ImUrX]

Closes #1

• removes behavior of erroring when no comment found and instead just returns an empty string (Discuss if it should be an Option or not)
• decoupled comment fetching behavior to just require the Vec<Attribute>
• makes an array of comments, being each index a different field
• exposes phf in the main crate for the macro to use
• makes a function with an static phf map which stores the field's name and the index to the field's comment
• half-do a test

[ImUrX]

Superseded by #3