Not null fields

[jhass]

Having to handle nil on every field access is a PITA. Support marking fields as not null, or maybe even make that the default and support marking them as nullable. Make the primary key not null by default

[drujensen]

yeah, crystal .not_nil! is very annoying. I wish they chose something like swift using just an ! or simpler syntax.

I have considered making the mapping more advanced and support nilable and other options that db's may support. It would be an options hash per field.

[drujensen]

closing since this is a duplicate of #57