Move to try_trait_v2 unstable feature

[atwam]

try_trait has now been removed (see rust-lang/rust#42327 (comment)) and replaced with try_trait_v2. Crate fails to compile with the latest nightly (because Try::Ok has been replaced with Try::Output.

I have switched to the new try_trait_v2. Please note that I have taken the liberty of removing try_fold_result and try_rfold_result, given try_fold and try_rfold are now stable (even though Try still requires try_trait_v2). I could re-instate them if you prefer (although I'm not sure whether to prefer wrapping a Result with a Try, or the other way around).