Minor optimizations for ZPure's runloop

[kyri-petrou]

I came across a few things in the ZPure runloop that could be optimized since it was initially rewritten about a year ago.

Main changes:

1. Since there's only 1 flag type, remove Flag and replace it with ClearLogOnError in order to avoid pattern matching on the flag type in the runloop
2. Use return Left(err) instead of throwing an exception and catching it later. This is more optimized as the C2 compiler doesn't optimize branches when an exception is thrown (even if it doesn't fill stack-traces). We saw this previously in ZIO where we used to throw an exception for yielding in the runloop (which was then replaced by returning null instead)
3. Remove a few redundant intermediate vals (which we previously needed just to call .asInstanceOf on).