Add generic default

[xsebek]

Is your feature request related to a problem? Please describe.

I want to write a dummy implementation of a value of an unknown type and not throw an exception.

I can not do that with undefined because Swarm is not lazy. It would have to be wrapped in either a function or delay.

Describe the solution you'd like

I would like to have a new keyword default : forall a. a, which will provide a default value of that type:

• 0: Int
• "": Text
• noop: Cmd t
• inl default: (t1 + t2)
• \_. default: t0 -> t1
• ...

Describe alternatives you've considered

I can write a definition like this:

def implementation_except_for_value = \value. [value=value, ...]

And provide some default at the call site, but this is not ergonomic.

Additional context

Admittedly, when #495 is implemented, I will not need dummy values as often.