Add type annotations to fix strong-mode errors

[natebosch]

• Add analysis_options so the analyzer runs in strong mode
• Add covariant BigInteger to many method arguments to fix errors
  where a function taking a specific type was overriding a method taking
  dynamic.
• Add specific type arguments to methods which use fields from the
  implementations that don't exist on BigInteger
• Restrict SDK to >1.22 to pick up covariant keyword

This is the minimal change to make the package strong mode clean, and
hopefully compatible with DDC.

[natebosch]

This should fix #39

[davidmorgan]

@kaendfinger any chance you could take a look at this please -- we are eager for the fix :)

Thanks.