Make Unit.constant public again.

[xbjfk]

Hello,

I'm really enjoying Rune. I'm making a rust program to run rune files where I need to access constants. I have a file like this:

pub const FOO = 999

And I want to access the value of FOO in Rust.

It seems this feature was added in #316 / 347967e, but then removed in #495 / 6cd6907 (28a1ae5 in PR) to make fewer things public.

The rune-cli can support --dump-constants as it's mostly in the rune crate.

Let me know if there is another way expected to do this from my application.