Add support for "try f" (when f is a function)

[d-torrance]

This returns a new compiled function closure that is equivalent to x -> try f x.

Closes #4184

[mahrud]

I guess in the current behavior, something like try(x -> x/0) never fails until you actually run it, or unless there's a syntax error in which case try doesn't prevent the error either, so maybe this is fine?

Does this also allow select(5, try (x -> (x-1)^1) else false) for instance? (meaning, try that function, and if it fails instead of null return false). I imagine we can combine it with other try syntax as well.

Also, this is tangential, but I've always wanted this to work:

i5 : apply(5, x -> if even x then continue else x)
stdio:5:29:(3): error: unhandled continue command

It's really annoying that I have to use return instead and then remove null entries.

[d-torrance]

Does this also allow select(5, try (x -> (x-1)^1) else false) for instance?

Not currently -- only try f is supported. I suppose implementing all the other variations of try wouldn't be too bad. I can give it a shot.