Replace stdout with a finalize function

[jeffcharles]

Here's a proposal for how to stop using stdout. This builds on using a singleton context. We remove the finalize function from the API we expose to partner code and create an exported finalize function for the host environment to call which will return a pointer and length to the output buffer. We can exclude calling this finalize function from the fuel and timeout limits.

A tradeoff is we lose performing a final validation of the WriteState in the partners' function. We could move that check to our finalize function or otherwise validate the state of the output vector on the host. That would change how an invalid output surfaces from a function trap to some other form of output error.

[jeffcharles]

Superseded by #99