try -- run code and handle fault of type `fallible.this`.

try is a type feature that can be called on any child of `fallible`
to run the code given as an argument and handle an error using the
`Unary` provided to `catch`.

This enables Java-like try-catch syntax as follows


or even


Note that the code is not executed unless `.catch` or `infix ||` is applied
to the result of the call to `try`.