Add trait that converts from DataWord to a custom word type

[mjhouse]

Users of this library will probably be transmitting words that, while they may use the same sync/parity bits, interpret the u16 word bodies in a specific way (not as bytes, &str etc). To support this, we need to:

• Provide a way of quickly creating a custom word
  • Initially thought this would be a trait but investigating proc-macro¹
  • Derive(Word) on a custom struct that has two attributes (data and parity) decorated
  • Proc macro crate could be a sub-crate of mil_std_1553b
• Provide access to the Field struct
  • Make the Field struct part of the public API
  • Update the Field struct to support fields greater than 8 bits
  • Add documentation and examples to the Field struct
• Update the Message::get method to be generic or add another, generic getter method.²
• Add a comprehensive example for defining and parsing custom words. Add standalone examples to project #54

After everything is implemented a user should be able to do the following:

#[derive(Word)]
pub struct CustomWord {
    /// the body of the word
    #[data]
    mydata: [u8;2],

    /// the parity bit of the word
    #[parity]
    myparity: u8,

    /// some other data
    othervar: u8,
}

impl CustomWord {

    /// Mask for parsing a custom flag from the leading bit
    pub const CUSTOM_FLAG: u16 = 0b1000000000000000;

    /// Field definition for the custom flag
    pub const CUSTOM_FLAG_FIELD: Field = Field::from(Self::CUSTOM_FLAG);

    /// Check the custom flag on the word
    pub fn is_custom_flag(&self) -> bool {
        Self::CUSTOM_FLAG_FIELD.get(self) == 1
    }

}

Footnotes

1. https://doc.rust-lang.org/reference/procedural-macros.html ↩

2. message.get(0).map(CustomWord::from).unwrap() ↩

[mjhouse]

Resolved by #66, I'll make a separate issue for the change to field.