Implements `Result` in fast-calls. Note that the approach here is
slightly different. Rather than store the last result in the `OpState`,
we put it into the `OpCtx` which saves us a lookup and lock in the error
case. We do not have to lock this field as it's guaranteed only one
runtime and thread can ever access it.
The fastcall path for many ops can avoid doing a great deal of work,
even for `Result` return values. In the previous iteration of `ops`, all
`Result`-returning functions would fetch and lock the `OpState`,
regardless of whether it was used or not.
