Add function to discard error from `result` and return `optional`

[jbencin]

It would be nice to have the equivalent of Rust's Result::ok(), which discards the error from a Result and return an Option

Currently, to do this you would have to write:

(match (some-parsing-function bytes)
   ok (some ok)
   err none)

Consistent with the syntax I've proposed here for other type conversion functions, I propose adding to-optional to convert result types into optional types:

(to-optional (some-parsing-function bytes))

[friedger]

Why is the type conversion to optional needed? Do you have some examples?