Figure out a way to convert Continuation to Env only once per continuation

[alaviss]

Converting to Env is extremely expensive due to how Nim's object conversion check is implemented.

Currently we perform one conversion per environment access, which gets pretty expensive (try compiling stash/performance with --objChecks:on).

[disruptek]

I think Nim has plenty of knobs and levers here and I don't really want to turn the checks off at all -- it just seems unnecessary and out of scope.

That said, we could just push object checks on during the initial whelping and then push them off around the continuation legs.