Option value design

[bvssvni]

Dyon has no null value, but uses some(val) and none(). It has a lot of common with result value, but there are a few differences.

• some(x)
• none()
• unwrap(some(x)) returns x
• none()? propagates an error, returning from the function

The type of option is opt (any type) and opt[T].

This is designed for:

• Explicitly declare when a value is optional
• Check when mutating a variable that it also is optional
• Convenient for debugging
• Common way of handling errors