Add examples

[mjhouse]

This will partially resolve #54 and #41

• Added inner 'mil_std_1553b_derive' crate that contains proc macros to implement Word and some conversion functions for a custom word.
• Added default feature flag derive that includes the 'mil_std_1553b_derive' crate.
• Added proc macros:
  • A derive macro for Word that requires that Default is also implemented and that there are two fields that are decorated with #[data] and #[parity], respectively. These two fields need types [u8;2] and u8 respectively.
  • An attribute macro called #[field(...)] that takes an identifier and a u16 bit mask, and adds a const mask and const field to the impl.
• Added two example projects that demonstrate a custom word and simple message construction.
• Made Field public
• Added get_as<T>() method of message
• Moved Word trait into its own file
• Added some extra &DataWord conversions

[github-actions]

Coverage

File	Coverage
All files	99%	✅
src/message/packets.rs	100%	✅
src/errors.rs	100%	✅
src/word/words.rs	98%	✅
src/fields.rs	100%	✅
src/message/messages.rs	98%	✅
src/word/enums.rs	100%	✅
src/flags.rs	100%	✅

Minimum allowed coverage is 95%

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