Struct-writer

Fixes
- Generate bit-mask correctly when parsing bit-fields

Fixes
- Handle bytes and str data types when parsing bytes

Features
- Convert from bytes to json using provided definitions
- Assert structures' size matches the total of the member sizes

Change
- Group definitions now require `size` attribute

Features
- Convert bit-fields into bytestrings

Features
- Add bit-field type to definitions
- Generate bit-field code from templates and definitions