wasi log context as optional

[guybedford]

I wonder if it might be worth making the "context" argument for wasi logging entirely optional?

This way, both global and contextual logging could be supported.

[sunfishcode]

This certainly makes sense from the perspective of "stderr", a concept shared by many programming languages, which is conceptually a "global " log.

I don't know if it generalizes beyond that though. I've sketched up a wasi-stderr API to cover the stderr case (and also fix the newline issue). If that makes sense, then perhaps wasi-logging should just focus on the contextual logging case?

[guybedford]

Assuming stderr can be fully relied upon for the general case in this way, then that distinction makes sense to me.

[sunfishcode]

Stderr now uses a dedicated wasi-stderr interface rather than using wasi-logging, so it no longer requires a context and no longer prints extra noisy output.