Field Attributes

For more control about which fields should be used by InterStruct or in which way they should be used, field attributes will be added.

The idea is to have serde-like configuration pattern. I.e.:

struct SomeStruct {
    #[inter_struct(rename="something_else", unchecked)]
    something: String,
}

For now several options are planned:

• unchecked InterStruct does some naive type checking by simply comparing the two types tokens.
  If the user is sure that the types are identical, unchecked will disable all of InterStruct's type checking.
• rename="OtherName" This allows to map a field to a differently named field on the target struct.
• ignore InterStruct will completely ignore this field.

The issue names no files, tests, or implementation entry points. Start by locating InterStruct's field-attribute parsing and type-checking code, then determine how serde-like attributes should represent unchecked, rename, and ignore. Done means all three listed options work for the shown struct syntax and are covered by tests.