Refactor Integer API

[hiltontj]

@Marcono1234 left some good suggestions on #98 that should be addressed:

• Extend API with more From implementations (feat: implement integer type to comply with spec #98 (comment))
• checked_abs does not need to return an Option, it will always be Some (feat: implement integer type to comply with spec #98 (comment))
• The API on the checked_* functions that take mut self is a bit odd, it may be better to take &self to avoid confusion and potential foot guns.